Answer: b.a categorical variable.
Step-by-step explanation:
In numerical variables, such as rainfall measured in inches or the number of sales in a day, the numbers have a numerical meaning. But a categorical variable is a category such as gender and national origin. It can be represented as a number, like the Social security number, but the numbers won´t have a numerical value. If your SSN is bigger than mine, it doesn´t mean that you are greater or at a different level on a scale, because the numbers are used as categories, not as mathematically meaningful.