In which sentence does an adverb clause modify a verb?
the flag, flapping in the wind as i attempted to do my studies, made concentration impossible.
i wanted to visit the museum while “the star spangled banner” was still being displayed.
we saluted because the flag had been raised.
the flag ceremony ended sooner than we expected.

Adverbs refer to the words which modify the verb, adjectives or adverbs, expressing a relation of a cause, time, place and so forth.

Whereas, an adverb clause refers to the group of words which works as an adverb. An adverb clause begins with the subordinating conjunction such as if, although, because and so forth. Lastly, it includes subject and predicate.

Therefore, the sentence which does an adverb clause modify a verb is, We saluted because the flag had been raised.

In the above sentence, "because the flag had been raised" modifies the verb "saluted."

cassius, casca, and their allies plant false documents to manipulate brutus to join their cause to remove caesar. after doing so, they visit brutus at night in his home to persuade him of their views. there they plan caesar's death. brutus is troubled but refuses to confide in his devoted wife, portia. on 15 march, caesar's wife, calpurnia, urges him not to go to the senate. she has had visionary dreams and fears the portents of the overnight storms.

c. barbarians with unexpected intelligence