There was a marked deterioration in his condition.

A. reformation

B. amendment

C. improvement

D. revision

Answer: Option C

Solution(By Examveda Team)

Deterioration : the process of becoming progressively worse.
Reformation : the action or process of reforming an institution or practice.
Amendment : a minor change or addition designed to improve a text, piece of legislation, etc.
Improvement : an example of improving or being improved.
Revision : the action of revising.

Antonym of Deterioration is Improvement