Which of the following best describes an exothermic reaction?

An exothermic reaction is characterized by the release of heat to the surroundings. During such a reaction, the system (the substances involved in the reaction) loses energy in the form of heat, which results in an increase in the temperature of the surroundings. This is a fundamental concept in thermochemistry, where energy changes accompany chemical processes.

In practical terms, examples of exothermic reactions include combustion reactions, like burning wood or fuel, where energy is released as heat and light. This release of heat can be measured and is often utilized in various industrial applications.

The other options do not accurately define an exothermic reaction. For instance, the idea of absorbing heat describes an endothermic reaction instead, where the system gains energy, leading to a temperature decrease in the surroundings. The production of gas or the requirement of constant temperature can occur in various types of chemical reactions but does not specifically categorize a reaction as exothermic.