General reactions:
* Response: This is a general term for any kind of reaction.
* Reaction: This is also a general term, often used in scientific contexts.
* Reply: This implies a verbal or written response.
* Answer: This is a more specific term for a response to a question.
* Retort: This implies a quick and sharp response, often in the form of a comeback.
Specific reactions:
* Shock: A sudden and strong feeling of surprise.
* Anger: A strong feeling of displeasure or hostility.
* Fear: A feeling of apprehension or anxiety.
* Joy: A feeling of great happiness.
* Disappointment: A feeling of sadness or dissatisfaction.
Other possibilities:
* Behavior: This refers to a person's actions in response to a situation.
* Action: This refers to any physical or mental activity.
* Outcome: This refers to the result of a reaction.
To choose the best noun for "react," consider the specific context of your sentence and the meaning you want to express.