In this sentence "look in the index for a mention of this type of error" which word is the subject; type, mention, (you), or index

The verb of the sentence - LOOK - is in imperative mood and it is a direct speech type sentence, in which the second person singular or plural is/are requested to do something. Imperative imply a second-person subject, and this can be omitted from the sentence.

The answer to this question: the subject of the sentence is (you)

D. subjects and predicates : )