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

Regulators

The correct answer is Option (3) – Regulators

Regulators:

Some organisms are able to maintain homeostasis by physiological (sometimes behavioural also) means which ensures constant body temperature, constant osmotic concentration, etc. 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 mammals have succeded other animals in due course of time is due to their ability to maintain a constant body temperature, they are able to survive in Antarctica as well as in Sahara desert.

Partial Regulators:

These try and regulate their internal environment by means of physiological mechanisms or behavioural attitude, but beyond a certain limit, they have to conform to the external environmental conditions for survival.

Conformers:

According to the inhabited area conformers change their body temperature or osmolarity of the body fluid. These organisms do not have any internal mechanism to regulate and no energy is spent.

Migrate : The organism can move away temporarily from the stressful habitat to a more hospitable area and return when stressful period is over.Many animals, particularly birds, during winter undertake long-distance migrations to more hospitable areas. Every winter the famous Keolado National Park (Bharatpur) in Rajasthan host thousands of migratory birds coming from Siberia and other extremely cold northern regions.