Many of the runners ____ incredibly fast.
The subject is "many" and many is plural.
Has would be wrong because you need a linking verb (the predicate is an adjective)
"Is" would be wrong because the subject is plural and "is" corresponds to a singular subject.
"Was" is incorrect because "was" corresponds to a singular noun.
Therefore "are" would be correct because it is a linking verb that corresponds to a plural noun.
