The correct option is C b
Let’s examine the sentence: “Language is a skill activity by which fine distinctions of meaning can be made for better understanding of behavior.” The sentence should use the preposition “through” in place of “by.” This is because “by” refers only to the superficial means of attaining the end while “through” refers to the entire process of attaining the end. Since, the sentence indicates that the end is to make out “fine distinctions of meaning,” we can understand that it is indicating to the process and not superficial methods. So, the correct sentence would be, “Language is a skill activity through which fine distinctions of meaning can be made for better understanding of behavior.” So, the error is in Part b of the sentence. Hence, Option B is the correct answer.