The hikers enjoyed the singing birds.

a.

participle

b.

gerund

c.

infinitive

I think that the answer is b. gerund but I'm not sure

Assuming the underlined word is 'singing,' the correct answer is going to be A. participle.

A participle and a gerund look the same - they both end in - ing. However, participles are used as adjectives (as is the case here), whereas gerunds are used as nouns. Infinitives always have the same form, which is TO + verb.