✅ The correct answer is A) "had been scrutinizing." This is the Past Perfect Continuous tense, which is used to describe an action that started in the past and continued up to another specific point in the past. The phrase "By the time the auditors arrived last Friday" sets the specific past point, and "for three weeks straight" emphasizes the duration of the action leading up to that point. The structure for Past Perfect Continuous is: Subject + had + been + Present Participle (V-ing).

❌ B) "has been scrutinizing" is the Present Perfect Continuous tense, used for actions that started in the past and continue up to the present. This is incorrect because the arrival of the auditors ("last Friday") clearly places the endpoint of the action in the past, not the present.

❌ C) "will have been scrutinizing" is the Future Perfect Continuous tense, used for actions that will continue up to a specific future point. This is incorrect as the context of the sentence is entirely in the past, not the future.

❌ D) "was scrutinizing" is the Past Continuous tense. While it describes an ongoing action in the past, it doesn't convey the duration of the action leading *up to* a specific past point from an *even earlier past point* with the same emphasis as the Past Perfect Continuous, especially when a precise duration ("for three weeks straight") is explicitly mentioned. The Past Perfect Continuous specifically highlights that the three-week period of scrutiny had been ongoing right up until the moment the auditors arrived.

✅ The correct answer is A) had been toiling. This sentence uses the Past Perfect Continuous tense, which is used to describe an action that started in the past, continued for a duration, and was still ongoing or had just stopped at another specific point in the past. The phrase "for three consecutive weeks" explicitly indicates this duration leading up to the arrival of the expedition.

The sentence structure for the Past Perfect Continuous is: Subject + had + been + V-ing (present participle).

❌ B) were toiling is in the Past Continuous tense. This tense describes an action that was ongoing at a specific moment in the past, but it doesn't emphasize the duration leading up to that point from an earlier time, which is crucial here with "for three consecutive weeks."

❌ C) had toiled is in the Past Perfect tense. This tense describes an action completed before another past action or focuses on the result. While it occurs before the expedition arrived, it doesn't emphasize the continuous nature and duration of the action up until that point as strongly as the continuous form does, especially with the explicit duration given.

❌ D) toiled is in the Simple Past tense. This tense describes a completed action in the past, without emphasizing its duration or its relationship as an ongoing action leading up to another past event.

✅ The correct answer is A) "will have been devising." This uses the Future Perfect Continuous tense (Subject + will have been + verb-ing), which is used to describe an action that started in the past, has been continuing, and will still be ongoing or completed at a specific point in the future, emphasizing its duration. The phrase "by 2030" indicates a future deadline, and "for at least three decades" highlights the continuous duration of the action leading up to that future point.

❌ Option B) "have been devising" is Present Perfect Continuous, indicating an action that started in the past and continues up to the present. This is incorrect because the sentence explicitly references a future point ("by 2030") for the completion or ongoing nature of the action's duration.

❌ Option C) "had been devising" is Past Perfect Continuous, describing an action that started in the past and continued up to another specific point in the past. This is incorrect as the sentence clearly points to a future time frame ("by 2030").

❌ Option D) "will devise" is Simple Future tense. While it refers to a future action, it does not convey the sense of an ongoing action that began much earlier and continues with emphasis on its duration up to a future point, which is critical given the phrase "for at least three decades."

✅ The correct answer is A) "will have been studying." This uses the Future Perfect Continuous tense, which is appropriate for an action that began in the past (after high school), continues through the present, and will continue up to a specific point in the future ("By the time he graduates next year"), emphasizing the duration ("for over six years"). The sentence structure for Future Perfect Continuous is: Subject + will + have + been + verb-ing (present participle).

❌ Option B) "has been studying" is in the Present Perfect Continuous tense, indicating an action that started in the past and continues up to the present moment. The phrase "By the time he graduates next year" clearly refers to a future point, making this tense inappropriate.

❌ Option C) "had been studying" is in the Past Perfect Continuous tense, indicating an action that started in the past and continued up to another specific point in the past. The context of Mark's graduation being "next year" places the event in the future, not the past.

❌ Option D) "will be studying" is in the Future Continuous tense, which describes an action that will be in progress at a specific time in the future. While Mark will indeed be studying, this tense does not emphasize the duration of the action leading up to that future point, which is crucial here with "for over six years."