The correct option is A decrease
The word 'augment' refers to making something greater by adding to it.
Option A, prohibit refers to formally forbidding someone from doing something.
Option B, decrease refers to making or becoming smaller or fewer in size.
Option C, surpass refers to something that exceeds or becomes greater than.
Option D, compensate refers tp paying someone for a loss or injury occurred.
Thus, according to the meaning, the word 'decrease' is the opposite of the word 'augment'.