Which of the following best illustrates the concept of a nominal variable?

Nominal variables are categorical data types that represent different categories or groups without any inherent order. The concept is best illustrated by the choice indicating the species of animals. Each species is a distinct category that does not have a ranking or order relative to the others. For example, species like cats, dogs, and birds represent different groups, but there is no sense in which one species can be considered higher or lower than another in a numerical or ordered sense.

The other choices do not fit the definition of nominal variables as well. Temperature in Celsius is a continuous variable that can be measured and ranked. Education level, while categorical, typically has an inherent order (e.g., high school, bachelor's, master's), making it an ordinal variable. Weight is a quantitative measure that can be expressed in numerical terms and can be compared in a straightforward manner, making it a ratio or interval variable rather than nominal. Therefore, species of animals accurately captures the essence of a nominal variable by exemplifying distinct groups without a defined order.