Many of the propositions he put up at the conference were ludicrous.

Answer: Option C

Ludicrous : so foolish, unreasonable, or out of place as to be amusing.
Obnoxious : extremely unpleasant.
Humorous : causing laughter and amusement; comic.
Praiseworthy : deserving approval and admiration.
Impractical : not adapted for use or action; not sensible or realistic.

Antonym of Ludicrous is Praiseworthy