Answer: B) who
Explanation: a pronoun is a word that can function as a noun phrase used by itself and that refers either to the participants in the discourse or to someone or something mentioned elsewhere in the discourse. Whom, whoever, whose and who are pronouns, whom should be used to refer to the object of a verb or preposition. Who and whoever should be used to refer to the subject of a sentence. Whose should be use to indicate belonging. In the given sentence the pronoun is referring to Nathan Heinz, which is the subject of the sentence, so it would be: Nahtan Heinz, who many consider a highly skilled vaulter, will be competing today.