Question

The hikers enjoyed the singing birds.

a.

participle
b.

gerund
c.

infinitive

Answer 1

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

Answer 2

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.