The influence of political broadcasts on elections is considerable.

Answer: Option C

Considerable : notably large in size, amount, or extent.
Inadequate : lacking the quality or quantity required; insufficient for a purpose.
Uninfluential : having little or no influence.
Negligible : so small or unimportant as to be not worth considering; insignificant.
Insignificant : meaningless.

Antonym of Considerable is Negligible