I listened, but I had no idea what he was ...... about.

Solution (By Examveda Team)

I listened, but I had no idea what he was talking about.

The verb "talking" is the most appropriate choice in this context as it fits naturally with the preposition "about".

"Saying" and "telling" do not pair well with "about" in this sentence structure.

"Discussing" implies a mutual exchange of ideas, which is not indicated here.