The word Mitigate means to make something (like pain, a problem, or a punishment) less severe, serious, or painful.
Let's look at the options:
\[\begin{array}{rl} \bullet & \text{Alleviate: To make a problem or suffering less severe. This is a synonym of Mitigate.} \\ \bullet & \text{Tranquilize: To make calm or sedate. While related to reducing agitation, it's not a direct antonym.} \\ \bullet & \text{Intensify: To become or make more intense or extreme. This is the direct opposite of making something less severe.} \\ \bullet & \text{Abate: To become less intense or widespread. This is a synonym of Mitigate.} \\ \end{array}\]
The word opposite in meaning to Mitigate is Intensify.