What term describes a chemical phenomenon where an organic molecule splits into simpler substances?

The term that describes a chemical phenomenon where an organic molecule splits into simpler substances is decomposition. Decomposition is a process in which complex organic compounds break down into simpler molecules, often with the involvement of microorganisms or through chemical reactions. This process is essential in nature for recycling nutrients and breaking down dead organic matter.

Combustion refers to a chemical reaction that typically involves the burning of substances in the presence of oxygen, resulting in the release of heat and light, along with the formation of water and carbon dioxide. Fermentation is a metabolic process that converts sugars to acids, gases, or alcohol, usually under anaerobic conditions, rather than breaking down organic molecules into their simplest forms. Oxidation involves the loss of electrons from a substance, often associated with reactions that may lead to decomposition but does not specifically define the simplification of organic molecules. Therefore, decomposition is the correct term for the process described in the question.