Help the detective complete her final report by choosing the grammatically correct word.

The suspect couldn't have stolen the diamond ___ he was locked in the police station at the time.

The correct answer is because.

We use the subordinating conjunction because to introduce an adverb clause of reason, which must contain a subject and a verb ("he was locked"). "Because of" and "due to" are prepositions that must be followed by a noun phrase, not a full clause. "Therefore" is an adverb that would require different punctuation (like a semicolon) to connect two independent clauses.