Answer :
The question is asking to complete the sentence ". _____ volunteers for this project will get credit for community service." and the options are: Whoever Who Whom Whomever. We can immediately reject "whom" and "whomever" since they are in indirect case. "who" would be acceptable, but it would sound then better to say "those that...." - therefore, the best answer is "whoever"
The correct answer is "whoever" in this sentence. The pronoun "whoever" is used as a subject pronoun since it it referring to the person or thing performing the action.
The options "whom" and "whomever" are not correct since they are "object pronouns" and not subject pronouns.
As regards option "who" it is also incorrect since it is used to introduce questions "Who is coming to work today?" or when referring to someone mentioned on the sentence before "The man who is dressed in red is my father". In the last sentence, the pronoun"who" is referring to "the man".
