The correct option is C sensible
The word 'ludicrous' refers to something so foolish and out of place or unreasonable.
Option A, obnoxious refers to something extremely unpleasant.
Option B, humorous refers to something causing laughter or amusement.
Option C, sensible refers to something done in accordance with wisdom or prudence.
Option D, impractical refers to something not sensible or realistic.
Thus, according to the meaning, the word 'sensible' is the opposite of the word 'ludicrous'.