What defines a ruminant animal?

A ruminant animal is specifically defined by having a complex stomach structure that allows it to digest fibrous plant material through a unique fermentation process. This type of digestive system includes multiple compartments, chiefly the rumen, reticulum, omasum, and abomasum, which work together to break down cellulose from plant materials. Ruminants, such as cows, sheep, and goats, exhibit behaviors like chewing cud, where they regurgitate and re-chew food to aid in its digestion.

The other options do not pertain to the essential characteristics of ruminants. For example, the focus on eating primarily grains or being carnivorous describes dietary habits rather than anatomical features. Similarly, the ability to chew food is a characteristic of many animals, including ruminants, but does not accurately define what makes an animal a ruminant. Thus, the complexity of the stomach structure is the defining trait of ruminant animals.