The correct option is C (c)
Let’s examine the sentence: “In spite of the fact that the meeting was about to end, he insisted to ask several questions.” The verb “insisted” is used with preposition “on” and not “to.” Hence, the correct version of the sentence would be, “In spite of the fact that the meeting was about to end, he insisted on asking several questions.” So, the error is in Part c of the sentence. Hence, Option C is the correct answer.