"Category" vs "Section" in this context

I have to choose the correct word in the following sentence:

In the 1730s, Swedish botanist Carl Linnaeus set out rules for classifying species, the most basic biological category/section.

According to the key, it is category. However, I do not have the slightest notion why section doesn't work either.

Here are the Cambridge dictionary definitions: category section

It may be category because you (Linnaeus) are categorizing species. That is, of a large group of disparate elements, you are sorting them into separate, stand-alone categories of species.

A section is typically understood to be a smaller part of a whole. Take for example a section in a book or a section in a restaurant. It is a "cut out" (etymology) of a larger piece (like intersect).