Answer:
The correct option is C.
Explanation:
The given binomial expression is

The first and last term of these type of expression are know as outer terms. In this expression first and last term are x and 4.
The middle terms (second and third term) of these type of expression are know as inner terms. In this expression inner terms are -1 and -3x.
Since -3x is an inner term, therefore option C is correct.