when it is reversible when it can be repeated when it requires heat to make it happen when energy is released by the process
I gave my students this multiple choice question: A student mixed baking soda with vinegar and observed that the reaction was endothermic. When is a reaction endothermic?

Only one answer is correct among these 4 choices:
- when it is reversible
- when it can be repeated
- when it requires heat to make it happen
- when energy is released by the process

Could you tell me which one is correct?

when it requires heat to make it happen