Step-by-step Solution:

The word "mitigate" means to make something less severe, serious, or painful. We are looking for its opposite.

• (a) Alleviate: This is a synonym, meaning to reduce severity.
• (b) Tranquilize: This means to calm, which is not directly opposite.
• (c) Intensify: This means to make more severe, which is the direct opposite.
• (d) Abate: This is another synonym, meaning to reduce in intensity.

Therefore, the word opposite in meaning to "mitigate" is "intensify".

Answer: (c) Intensify