Question

complete the sentence.
proboscis monkeys are social animals, ______________, and eat mainly leaves and seeds.
swim proficiently
proficient at swimming

Explanation:

To maintain parallel structure, the phrase should match the form of "social animals" (adjective + noun) and "eat mainly leaves..." (verb phrase). "proficient at swimming" is an adjective phrase ("proficient") + prepositional phrase, which is parallel to "social animals" (adjective + noun). "swim proficiently" is a verb phrase, which does not match the structure of "social animals". So the correct choice is "proficient at swimming".

Answer:

proficient at swimming