Though plants, fungi, and prokaryotes all have cell walls, we place them in different taxa. Which of these observations comes closest to explaining the basis for placing these organisms in different taxa, well before relevant data from molecular systematics became available?

Due to other differences.

Explanation:

The plants, fungi, and prokaryotes all have cell walls, we place them in different taxa because of the other differences such as mobility, mode of nutrition etc. Plants and fungi are not mobile means can not move from one place to another whereas prokaryotes are mobile. Fungi and prokaryotes are heterotrophs means that feed on other organism whereas plants are autotrophs means make their own food. There is also difference in their cell wall i.e. the cell of plants is composed of cellulose, the cell wall of prokaryotes especially bacteria is composed of peptidoglycans and the cell wall of fungi is composed of chitin.

geographic isolation