General synonyms:
* Answer
* Reply
* React
* Rejoin
* Counter
* Retort
* Acknowledge
More specific synonyms:
* Affirm (agree)
* Deny (disagree)
* Confirm (verify)
* Refute (disprove)
* Comply (obey)
* Yield (submit)
The best synonym to use will depend on the specific context of the sentence.
For example, "respond to a question" could be replaced with "answer a question," while "respond to a request" could be replaced with "comply with a request."