Change the narration: I said to Riaz "Who are you waiting for?"

I asked Riaz that for whom you are waiting.

I asked Riaz who he was waiting for.

I asked Riaz whom for he is waiting.

None of these

I asked Riaz who he was waiting for.

The question "Who are you waiting for?" changes to "who he was waiting for" in indirect speech.

The tense shifts from present continuous to past continuous, and "you" changes to "he".

No question mark is used in indirect speech.