Answer:
c. The students became quiet when the principal walked into the room.
Explanation:
A complex sentence is a sentence that has one independent clause and at least one dependent clause. An independent clause is a full sentence but a dependent clause needs an independent clause to be taken as a full sentence. Also, in a complex sentence, the dependent clause has to start with a subordinating conjunction.
Among the given sentences, sentence c is a complex sentence as it contains the subordinating conjunction "when". It also has one independent clause with a dependent clause.
Thus, the correct answer is option c.
