✅ In both sentences, 'since' functions as a subordinating conjunction, connecting a dependent clause to an independent clause.

In sentence I, 'since he started high school' is a dependent clause specifying the time period from which the main clause's action began, making 'since' a subordinating conjunction of time.

In sentence II, 'since the library was closed' is a dependent clause explaining the reason for the action in the main clause, thus 'since' acts as a subordinating conjunction of reason or cause.

❌ Option A is incorrect because 'since' is never a coordinating conjunction; it always introduces a dependent clause, unlike coordinating conjunctions (FANBOYS) which connect grammatically equal independent clauses or other equal elements.

❌ Option B is incorrect because in sentence I, 'since' is followed by a full clause ('he started high school', containing a subject and verb), not just a noun phrase, meaning it functions as a conjunction, not a preposition.

❌ Option D is incorrect as 'since' in sentence I introduces a complete clause, not merely modifying a verb or adjective as an adverb would; additionally, 'since' does not function as a coordinating conjunction.

✅ In option C, "However much he trained" is an adverbial clause of concession, meaning "no matter how much he trained."

Here, "however" introduces this dependent clause, making it a subordinating conjunction that links the dependent clause to the independent clause "he couldn't beat the current champion."

❌ In options A, B, and D, "however" is used as a conjunctive adverb, not a subordinating conjunction.

It connects two independent clauses, often requiring a semicolon before it and a comma after, indicating a contrast or concession between the two complete thoughts.

Conjunctive adverbs like "however" modify the second clause while signaling a relationship with the first, but they do not integrate a dependent clause into a main clause in the way a subordinating conjunction does.

✅ "Since" functions as a subordinating conjunction in option D, introducing the dependent clause "Since the artifact was extremely fragile." This clause clearly provides the reason for the independent clause "it required specialized handling," correctly forming a complex sentence that expresses the intended causal relationship.

❌ "Therefore" in option A is a conjunctive adverb, not a conjunction. It connects two independent clauses, often forming a compound sentence with a semicolon, but it does not make one clause grammatically dependent, thus failing to create a complex sentence.

❌ "And" in option B is a coordinating conjunction, which joins two independent clauses to form a compound sentence. While grammatically correct, it merely adds information and does not explicitly convey the specific "reason why" relationship desired, which is to explain why the handling was necessary.

❌ "Because" in option C is a subordinating conjunction, but its placement ("The artifact was extremely fragile because it required specialized handling") reverses the logical meaning. It incorrectly implies that the artifact's fragility was *caused by* the specialized handling, rather than the handling being a *consequence of* its fragility.

✅ The word "because" is a subordinating conjunction that introduces the dependent clause "her previous grades were exceptional," which explains the *reason* why she managed to pass the course, thus indicating causality.

❌ "Although" is a subordinating conjunction, but it introduces a clause of *concession* ("despite the fact that"), not causality. It sets up a contrast with the main clause.

❌ "However" is a conjunctive adverb, not a subordinating conjunction. It connects two independent clauses (often requiring a semicolon before it) and expresses contrast, but it does not introduce a dependent clause that modifies or completes the main clause.

❌ "Still" is an adverb that modifies the verb "managed" or the entire clause, indicating continuation or emphasis. It does not function as a conjunction to connect clauses or phrases.

✅ 'Although' is a subordinating conjunction that introduces the dependent clause 'Although the initial plan was ambitious'. This clause expresses a concession or contrast to the main, independent clause 'the team successfully adapted it to the unforeseen challenges', forming a complex sentence.

❌ Option A is incorrect because 'Although' introduces a dependent clause, not an independent one, and the relationship is one of contrast, not addition. Coordinating conjunctions like 'and' or 'but' join independent clauses.

❌ Option C is incorrect as 'Although' connects clauses, not just parallel phrases, and its primary purpose here is to show concession, not sequential order.

❌ Option D is incorrect because this sentence is a complex sentence (one independent and one dependent clause), not necessarily a compound-complex sentence, which requires at least two independent clauses and one or more dependent clauses. 'Although' directly subordinates one clause to another, not linking two dependent clauses to an independent one in this manner.