General:
* Response: This is the most common and direct noun for "respond."
* Reaction: This emphasizes the effect of something on a person or thing.
* Reply: This implies a verbal or written answer to a question or statement.
* Answer: This implies a direct response to a question.
* Feedback: This emphasizes information given in response to a request for evaluation.
More specific:
* Retort: This implies a sharp or witty response.
* Rebuttal: This implies a response that refutes an argument.
* Counterargument: This implies a response that opposes an argument.
* Acknowledgement: This implies a simple confirmation of receipt.
The best choice depends on the specific situation and the nuance you want to convey.