What food group is flour from?

Flour is a carbohydrate, it is mainly made up of carbon and hydrogen hence its name.

Sugars are also in this group, but they are more simple and contain only one or a few subunits so they ar monosaccharides or disaccharides or even oligosaccharides, whereas starches, which is what flour is, contains many subunits, or saccharides that is why it is also called a polysaccharide.

This is a more detailed answer in case you were not referring to the food pyramid, but to macromolecules.