Define an endothermic reaction.

An endothermic reaction is defined as a reaction that absorbs heat from its surroundings. This means that during the process, the system takes in energy in the form of thermal energy, leading to a decrease in temperature of the surrounding environment. Common examples of endothermic reactions include photosynthesis and the melting of ice, where energy is required to break bonds or facilitate the reaction.

In contrast, reactions that release heat to the surroundings indicate exothermic processes, which are not applicable to the definition of endothermic reactions. The notion of a reaction occurring without heat exchange refers to isothermal processes but does not fit the characteristics of endothermic reactions specifically. Additionally, while all chemical reactions require activation energy to initiate, this characteristic does not distinguish an endothermic reaction from others since activation energy can be involved in both endothermic and exothermic reactions. Therefore, the focus on heat absorption clearly identifies option C as the accurate definition of an endothermic reaction.