MITIGATE :

Answer: Option B

Abate: (Of something unpleasant or severe) become less intense or widespread.
Aggravate: Make (a problem, injury, or offence) worse or more serious
Allay: Diminish or put at rest (fear, suspicion, or worry).
Alleviate: Make (suffering, deficiency, or a problem) less severe.
MITIGATE : Make (something bad) less severe, serious, or painful.

Antonym of MITIGATE is Aggravate.