In his lust for wealth, he had reached the zenith of criminal activity.

Answer: Option A

Acme : the point at which something is at its best or most highly developed.
Nadir : the lowest or most unsuccessful point in a situation.
Bottom : the lowest point or part of something.
End : a final part of something, especially a period of time, an activity, or a story.
Zenith : the time at which something is most powerful or successful.

Synonym of Zenith is Acme