Which was the main criterion used by Whittaker for classification?

Correct answer: Complexity of cell structure and mode of nutrition

Correct answer: B. Whittaker used meaningful biological criteria, especially cell structure and complexity, body or thallus organisation, mode of nutrition, reproduction, and evolutionary relationships. Cell structure separates prokaryotes from eukaryotes, while mode of nutrition helps distinguish organisms that make food, ingest food, or absorb nutrients. These criteria provide a much stronger scientific basis than superficial appearance. Option A, colour, can vary and is not a fundamental classification criterion. Option C, geographical distribution, tells where organisms live but does not define their major biological group. Option D, size alone, is also unreliable because unrelated organisms can be large or small. Memory cue: remember “cell structure + nutrition” as the core pair for Whittaker.