The correct option is
C Withdraw
The word 'reiterate' refers to saying something again or for a number of time, typically for emphasis or clarity.
Option A, withdraw refers to discontinuing or no longer providing something that was previously supplied. Thus, according to the meaning of the given word, 'withdraw' will be the correct antonym to 'reiterate'.
Option B, rectify refers to correcting something or putting it right.
Option C, affirm refers to stating something emphatically or publicly.
Option D, acknowledge refers to accepting or admitting the existence of something.