General:
* Answering
* Replying
* Reacting
* Retorting
* Counteracting
Formal:
* Responding (this is still a perfectly acceptable formal word)
* Reiterating
* Affirming
* Acknowledging
* Confirming
Informal:
* Chiming in
* Firing back
* Coming back
* Jumping in
* Shooting back
Specific Actions:
* Agreeing
* Disagreeing
* Explaining
* Clarifying
* Justifying
The best word to use will depend on the specific situation.