Answer:
The correct form to rephrase the sentence in the exercise using a direct object pronoun is:
Explanation:
The direct objects in Spanish are the words on which the used verb falls, these are, using the personal pronouns as guide:
How you can see, the direct object could replace a particular noun, in the sentence: Yo veo a los voluntarios, "los voluntarios" corresponds to the personal pronoun they or "ellos" in Spanish, by this reason, not to use that noun again, we use the direct object "los".