When presented with only circumstantial evidence, a juror tends to decide a case according to his or her gut instinct, which are not formally sanctioned or prohibited means of reaching a verdict.

(A) which are not formally

(B) which are not a formally

(C) which is not a formally

(D) which is formally not a

(E) which is not formally

OA: C

Why do we need "a" article in option C.