The reason he gave, _____, was most unconvincing.

Analyze the question

The sentence is: "The reason he gave, _, was most unconvincing." We need a phrase that correctly completes the noun phrase "the reason he gave" by telling us what the reason was.

Concept

The noun "reason" is normally followed by a that-clause (a noun clause) that states the reason itself: "The reason is that he had no money." The pattern "reason... because/since/for" is considered redundant or incorrect in standard English, because "reason" already carries the idea of "because."

Testing each option

• A. "which he had no money" is ungrammatical. "Which" has no function in the clause, because "he had no money" is already complete.
• B. "since he had no money" is an adverbial clause of reason. It would make "reason" and "since" say the same thing twice, and it does not state the content of the reason as a noun clause.
• C. "that he had no money" is a noun clause that gives the content of the reason. It fits: "The reason he gave, that he had no money, was most unconvincing."
• D. "for he had no money" uses "for" as a conjunction. It is awkward and redundant here, and it is not the standard way to complete "reason."

Answer

The correct option is C.

Tip

Remember: "The reason... is that...", never "the reason... is because." After "reason," use that. The same applies to similar nouns like "fact," "idea," and "belief" (the fact that..., the idea that...).