Choose the answer that identifies the gerund phrase.

The ringing doorbell was distracting him from answering the phone.

Ringing doorbell

was distracting <

Distracting him from answering the phone

Answering the phone

The gerund phrase functions as a noun, not a verb: here "was distracting" is a verb, so it can't be part of the Gerund phrase.

Ringing has a function of an adjective, so also not.

The correct answer is "answering the phone" - it could be substituted by "it": from it, so we know that it functions as a verb