A qualitative variable, also known as a categorical variable, refers to a type of data that can be divided into distinct categories that do not have a numerical value. In this case, blood type serves as a perfect example of a qualitative variable because it categorizes individuals into groups like A, B, AB, or O, without any inherent ranking or numerical relationship among them.
The other choices present quantitative variables, which are expressed numerically and can be measured on a continuous scale. For instance, age and height are measurements that involve numeric values and can provide a range of data points. Similarly, income is also a measurable quantity that can take on various numerical values, reflecting the amount of money an individual earns. These quantitative variables are distinctly different from qualitative variables, reinforcing the appropriateness of blood type as the correct example of a qualitative variable in this context.