The word "augment" means to make something greater by adding to it; to increase. We are looking for an antonym, which means a word with the opposite meaning.
\[\begin{array}{rl} \bullet & \text{(A) Aggrandize: To increase the power, status, or wealth of. (Synonym)} \\ \bullet & \text{(B) Reinforce: To strengthen or support, especially with additional material. (Synonym)} \\ \bullet & \text{(C) Curtail: To reduce in extent or quantity; impose a restriction on. (Antonym)} \\ \bullet & \text{(D) Inflate: To increase by a large or excessive amount. (Synonym)} \\ \end{array}\]
The word with the opposite meaning to augment (increase) is curtail (reduce).