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.

as the gila monster consumes the eggs or young of the desert cottontail and gambel's quail, thus its main food supply is only accessible for a restricted time duration every year. however, for this, the gila monsters exhibit three adaptations, which elevate their survival rate in the desert condition.  

first, is that they possess huge bodies, and can consume greater concentrations of food during a small period. secondly, they require to feed infrequently as they can accumulate fat in their tails. ultimately, their rate of metabolism is very low, so that they can burn their reservoirs of fat gradually.