Qualitative variables in theory

While R can tell us how variables are currently stored, we also need a conceptual understanding of how things "should" be stored. For example, what if you had a variable that represents the number of children someone has, such as, 1, 3, or 5, but one entry is "two"? The whole variable will be a character vector (string) and you'll need to change it to a numeric variable.

Which of the following is not a qualitative variable?