Question

The scientists knew instantly and without a doubt that they had made an amazing discovery. Which adverb modifies a verb?

Answer 1

The adverb that modifies the verb is instantly. Adverbs usually end in ly, so that should already be a clue, but the main reason is because the question is how fast did the scientists know? They knew instantly. 

Answer 2

The adverb that modifies a verb is instantly.

Step-by-step explanation:

An adverb is a word that describes where, when, or, in this case, how an action took place. In this sentence, the adverb is modifying a verb, because it is making clear how they scientists knew, but an adverb could also serve as modifier of an adjective or another adverb.