Question

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

Answer 1

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

Answer 2

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.