There was a marginal increase in his pay.

Answer: Option C

Marginal : minor and not important; not central.
Unforeseen : not anticipated or predicted.
Negligible : so small or unimportant as to be not worth considering; insignificant.
Significant : sufficiently great or important to be worthy of attention; noteworthy.
Peripheral : relating to or situated on the edge or periphery of something.

Antonym of Marginal is Significant