A substance which may alter environmental constituents or cause pollution is referred to as

The correct option is Pollutant.

To understand why this is the correct answer, let us break down the definition of the term and compare it with the other options provided:
1. Pollutant: A pollutant is any substance, energy (such as heat, light, or noise), or force introduced into the environment that has undesirable effects, or adversely affects the usefulness of a resource. In simple terms, any agent that alters the natural composition of the environmental constituents or directly causes pollution is defined as a pollutant. Examples include chemical waste, plastics, pesticides, and excess carbon dioxide.
2. Decomposer: These are organisms (like bacteria and fungi) that break down dead or decaying organic matter. They play a vital role in recycling nutrients back into the ecosystem rather than altering constituents negatively or causing pollution.
3. Radiator: A radiator is an object or substance that emits energy in the form of electromagnetic waves or radiation (for example, heat). It is not defined as a substance that alters environmental constituents to cause pollution.
4. Reducer: In chemical terms, a reducer (or reducing agent) is a substance that loses or donates electrons to another chemical species in a redox chemical reaction. In ecological terms, it does not specifically refer to a substance causing environmental pollution.

Therefore, any substance that may alter environmental constituents or cause pollution is precisely referred to as a pollutant.