Which sentence contains a gerund? Sally is fishing on the lake with her father and brothers. Marcus is supposed to be joining the discussion later today. My cat was not interested in the dangling string. Her chorus director does not appreciate Angela's fidgeting.

I'd have to go with B

An explanation to that is, A gerund is a noun formed with a verb ending in ing. Words like swimming, tying, dreaming, eating, fishing, and drinking can all be used as gerunds.

Here are 2 examples I found while googling:

"Blowing bubbles on a windy day is a fun activity for children."

Blowing bubbles on a windy day is the subject of the verb is.

"Ethan narrowly avoided driving off the cliff."

Driving off the cliff is the direct object of the verb avoided.