The correct option is C (C)
Let’s examine the sentence: “If I had known this earlier I will have helped him.” The sentence is in the form of the second conditional. This sentence talks about the second conditional that is used to express imaginary and impossible situations. So, second conditional used forms of verbs in past tense. Hence, the verb “will” in the sentence would be replaced by its past tense form, i.e. “would.” Hence, the correct sentence would be, “If I had known this earlier I would have helped him.” As the error is in part c of the sentence, Option C is the correct answer.