Formal:
* Answered
* Responded
* Retorted (implies a sharp or witty answer)
* Rejoined (suggests a continuation of a conversation)
* Countered (indicates a disagreement or objection)
Informal:
* Said
* Answered back
* Chimed in (implies joining a conversation)
* Shot back (suggests a quick and forceful answer)
* Fired back (similar to "shot back")
Other options:
* Remarked
* Observed
* Stated
* Declared
* Affirmed
* Conceded (implies agreement)
The best synonym for "replied" will depend on the specific situation and the meaning you want to convey.