One who is neither intelligent nor dull

A. Tolerable

B. Commoner

C. Mediocre

D. Diligent

Answer: Option C

Solution(By Examveda Team)

One word substitute is Mediocre.

Tolerable: able to be endured.
Commoner: one of the ordinary or common people, as opposed to the aristocracy or to royalty.
Mediocre: of only average quality; not very good.
Diligent: having or showing care and conscientiousness in one's work or duties.