✅ The correct answer is A) "should have been installed." The phrase "should have + past participle" expresses an unfulfilled obligation or expectation in the past, perfectly matching the context of an update that was expected but couldn't be installed last night. The passive voice ("been installed") is also correctly used because the security update is the recipient of the action, not the performer.

❌ Option B) "must have installed" uses "must have + past participle" which indicates a strong deduction about a past event, not an unfulfilled obligation. Moreover, "installed" is in the active voice, incorrectly suggesting the "security update" performed the action itself.

❌ Option C) "could be installing" indicates a present or future possibility or continuous action. This tense and meaning do not fit the past time reference ("last night") or the context of an unfulfilled past event. It also uses the active voice, which is grammatically inappropriate for the subject.

❌ Option D) "would have installed" is typically used in conditional perfect structures to describe a hypothetical past action (e.g., "If I had been there, I would have installed it"). It does not convey the meaning of an unfulfilled past obligation or expectation as accurately as "should have." Furthermore, it is in the active voice, which is incorrect for "security update" as the subject.