Answer: In sentence D. The veteran appeared proud as he saluted the flag, the adverb clause modifies an adjective.
Explanation: An adverb clause is a dependent clause that functions as an adverb. It cannot stand alone as a sentence because it does not express a full thought. Furthermore, an adverb clause can be used to describe an adjective, a verb or another adverb. For example, in sentence D., the adverb clause is "as he saluted the flag" since it is being introduced by the subordinating conjunction "as" and it does not express a full idea. Moreover, this clause is modifying an adjective: "proud", which is immediately before it.