Mammals are able to survive in Antarctica or in the Sahara desert as…

The correct answer is Option (3) → Regulators

Option (1) Conformers — Incorrect. Conformers cannot keep their body temperature constant; it changes with the surroundings. About 99 per cent of animals are conformers.

Option (2) Partial regulators — Incorrect. Partial regulators can regulate only over a limited range of conditions, beyond which they conform.

Option (3) Regulators — Correct. Mammals keep a constant body temperature (homeostasis), so they live in both extreme cold and extreme heat.

Option (4) Migrants — Incorrect. Migration means moving away from a stressful habitat for a time; animals that live in Antarctica or the Sahara stay there.

Note: This question is from the topic “Responses to Abiotic Factors” in Chapter 13, Organisms and Populations, of the pre-rationalised NCERT book. This portion is not in the present NCERT book, where the chapter is now Chapter 11, Organisms and Populations. The extract below is from the pre-rationalised edition.

NCERT Extract (Pre-rationalised edition, Chapter 13, Organisms and Populations):
Responses to Abiotic Factors – “… All birds and mammals, and a very few lower vertebrate and invertebrate species are indeed capable of such regulation (thermoregulation and osmoregulation). Evolutionary biologists believe that the ‘success’ of mammals is largely due to their ability to maintain a constant body temperature and thrive whether they live in Antarctica or in the Sahara desert.”
Responses to Abiotic Factors – “… Considering the benefits of a constant internal environment to the organism, we must ask why these conformers had not evolved to become regulators.”