The correct option is B alleviate
The word 'aggravate' refers to making a problem worse or more serious.
Option A, abbreviate refers to shortening of something.
Option B, alleviate refers to making a suffering or problem less severe.
Option C, advocate refers to publicly supporting or recommending a particular cause or policy.
Option D, appreciate refers to recognizing the full worth of something.
Thus, according to the meaning, the word 'alleviate' is the opposite of the word 'aggravate'.