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

The correct answer is D. impractical.

Ludicrous means “extremely foolish or ridiculous.” In the context of the question, it is clear that the speaker is not praising the propositions, but rather criticizing them for being unrealistic or unworkable.

The other options are not as relevant to the meaning of the word “ludicrous.”

• Obnoxious means “extremely unpleasant or annoying.” This does not seem to be the speaker’s intention, as they are simply stating that the propositions were not practical.
• Humorous means “causing laughter or amusement.” This is not the speaker’s intention, as they are not trying to be funny.
• Praiseworthy means “deserving of praise.” This is not the speaker’s intention, as they are not praising the propositions.