Concept:An endothermic reaction absorbs heat energy from its surroundings.
Explanation:During an endothermic reaction, heat is taken in from the surroundings.
This makes the reaction vessel feel cool to the touch.
So the correct sign is option A.
Option B is wrong because a negative enthalpy change means an exothermic reaction.
Option C is wrong because greater bond-forming energy than bond-breaking energy releases heat.
Option D is wrong because it also indicates an exothermic process.
Answer:A. reaction vessel feels cool during the reaction