The correct option is C c
Let’s examine the sentence: “The reason why the plane crashed as soon as it took off is the failure of one of the engines.” The sentence has two parts, “The reason why the plane crashed as soon as it took off” and “is the failure of one of the engines.” The first part states the effect of the action and the second part states the cause of the action. So, the conjunction “because” should be used to join the two parts and create the relationship of cause and effect. The correct sentence would be, “The reason why the plane crashed as soon as it took off is because of the failure of one of the engines.” So, the error is in Part C of the sentence. Hence, Option C is the correct answer.