The correct answer is: D. maintained it proper.
The sentence should be: “Your machine would not have given you so much trouble if you had maintained it properly.”
The word “proper” is an adjective, and it should be used to modify a noun. In this case, the noun is “maintenance.” The correct way to use the word “proper” is to say “proper maintenance.”
The word “proper” can also be used as an adverb, but it is not used in this way in the sentence. An adverb modifies a verb, an adjective, or another adverb. In this case, the verb is “maintained.” The correct way to use the word “proper” as an adverb is to say “maintained properly.”
Therefore, the correct sentence is: “Your machine would not have given you so much trouble if you had maintained it properly.”