Which is the subject of this sentence? Look in the index for a mention of this type of error.

a. index

b. (you)

c. mention

d. type

The correct answer is B, it is an understood 'you' which is not explicitly stated in the sentence, but it is understood that it is the subject of the sentence.

A is the object.

C is the verb.

D is object of preposition.

The subject in the sentence is index/A because that's the most important part its the main part