Concept: Endothermic reactions are defined by the net absorption of heat energy from the surroundings.Explanation: In an endothermic reaction, the system gains heat and the surroundings lose heat, so the temperature of the surroundings decreases.This means heat energy is absorbed by the reacting system.High activation energy is not a necessary feature, a catalyst is not required, and the reaction does not have to be reversible.Answer: D. Heat energy is absorbed