The biological association between bacteria and protozoa in the rumen of cattle is an example of

The rumen is the first compartment of the stomach in ruminant animals such as cattle. It houses a dense community of microorganisms, including bacteria and protozoa, that help break down cellulose and other complex plant materials that the cow cannot digest on its own.

This relationship is symbiosis (specifically mutualism), because both parties benefit. The bacteria and protozoa gain a warm, moist, anaerobic environment with a constant supply of food from the plant material the cow ingests. In return, they produce enzymes such as cellulase that digest cellulose into simpler compounds like volatile fatty acids, which the cow absorbs and uses as a major energy source. The microorganisms also synthesise vitamins (such as B-vitamins) and microbial protein that the cow digests further along the alimentary canal.

This is not commensalism, because commensalism involves one organism benefiting while the other is neither helped nor harmed. Here, both parties clearly benefit. It is not parasitism, because neither organism is harmed. It is not predation, because neither organism kills and consumes the other as prey.

Exam tip: Whenever a question describes two different organisms living together with mutual benefit, the correct ecological term is symbiosis (mutualism). The rumen of cattle is the classic JAMB example of this association.