The best definition of the term pollutant is Choice B: any substance that is harmful to the environment. This definition captures the essence of what pollutants are, as they negatively affect air, water, soil, and living organisms.
While Choice A mentions that pollutants are things added by humans, not all substances introduced through human activity are necessarily harmful. Some can even be beneficial or neutral. Choice C suggests that pollutants are just waste, but not all waste is harmful; some can be recycled or processed safely.
In summary, a pollutant is specifically recognized for its harmful effects on the environment, making choice B the most accurate definition.