electric current a net loss of mass thermal insulation an exothermic reaction
I gave my students this multiple choice question: A hot pack is a product that releases heat when chemicals inside of it interact. What is taking place inside an active hot pack?

Only one answer is correct among these 4 choices:
- electric current
- a net loss of mass
- thermal insulation
- an exothermic reaction

Could you tell me which one is correct?

an exothermic reaction