An example of a nominal variable is:

A nominal variable is a type of categorical variable that represents discrete categories or groups without a specific order. In this case, gender serves as a clear example of a nominal variable because it categorizes individuals into distinct groups, such as male and female, without implying any sort of ranking or order. The categories within a nominal variable are merely labels that classify observations into groups based on qualitative traits.

In contrast, income level is typically a ratio variable, as it quantifies the amount of income and reflects differences in magnitude. Temperature is also a quantitative variable since it can be measured on a scale and represents a continuous variable with an inherent order. Exam scores, similarly, are ordinal or interval variables, as they reflect a ranking or degree of achievement on a common scale, allowing for comparisons of magnitude between scores.

Thus, the distinctive characteristic of gender as a nominal variable underscores its classification function without any inherent ordering, making it the correct choice in this context.