Some army units were decimated during the surprise attack.

The correct answer is D. destroyed.

To decimate something means to destroy a large part of it. In the context of the question, the army units were destroyed during the surprise attack. This could mean that they were killed, wounded, or captured.

The other options are incorrect because they do not accurately describe the meaning of the word “decimate.” To denounce something means to criticize it publicly. To capture something means to take it prisoner. To defeat something means to win a victory over it.