Concept:The phrase set off by commas should restate or identify “the reason.”Explanation:A clause beginning with “that” can act as an appositive noun clause.“That he had no money” directly names the reason he gave.The other options use “which,” “since,” or “for,” which cannot form this type of noun clause in apposition.So the correct sentence becomes: “The reason he gave, that he had no money, was most unconvincing.”Answer:C. that he had no money