Select the word or words that would correctly complete the following sentence. the

The word that would correctly complete this sentence is "older".

Explanation: The sentence provided includes a comparative structure, that is to say that it establishes a difference between two objects by means of comparison. A comparative structure always includes a comparative adjective and it never presents a superlative adjective. For that reason, "oldest", which is a superlative adjective, cannot complete this sentence. In that way, the comparative form of "old", which is "older" and not "more older" because "old" is a one-syllable adjective, is the word that fills in the blank.

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.