Step-by-step Solution:

The word "augment" means to increase, enlarge, or make something greater.

Now, let's analyze the options:

A. Aggrandize – This means to enhance, increase, or make something appear greater, which is similar to "augment," not an antonym.
B. Reinforce – This means to strengthen or support, which also aligns with the meaning of "augment."
C. Curtail – This means to reduce, shorten, or limit, which is the opposite of "augment," making it the correct antonym.
D. Inflate – This means to expand or increase, which is again similar to "augment."
Thus, the correct antonym of "augment" is C. Curtail.