In five-kingdom system, the main basis of classification is

Explanation

In Whittaker's five-kingdom system of classification, the main basis is mode of nutrition, which distinguishes autotrophs (plants), heterotrophs by ingestion (animals), heterotrophs by absorption (fungi), and mixed types (protists, monerans). Other factors like mode of reproduction, complexity of behavior, and number of chromosomes were not used as the primary basis, since they are either variable, not universal, or less significant at the kingdom level.