The correct option is
D always maintain a stable internal environment irrespective of the changes in the external environment
An organism is any living system, that can function as an individual entity. As the conditions of temperature, humidity, etc. in every habitat varies throughout the year, organisms adapt themselves to the changing environment by developing various mechanisms.
Organisms such as birds and mammals are considered as regulators who maintain a constant internal environment (homeostasis) at all times irrespective of the changes in the external environment. This stability is shown in terms of a constant body temperature, osmotic conditions, etc. Hence option b is correct.
Maintaining a constant internal environment is important because the metabolic functions within body of organism require optimal conditions of temperature, pH, osmotic concentrations, etc. This helps regulators to perform better in various environmental conditions and is the key to their survival in diverse ecological habitats.
The human body keeps itself cool by sweating when the external temperature is high, or shivers in winters to generate heat. Similarly, birds can also maintain constant body temperature in colder surroundings by generating heat whereas in warmer surroundings, birds tend to keep their body temperature constant by cooling down the body by loss of water.
Organisms which do not maintain a stable internal environment and modify their internal environment to conform to the changes in the external environment are known as conformers. E.g., reptiles, fishes, plants, etc. Hence option a is incorrect.
Some organisms undergo sleeping or a period of inactivity to escape stressful conditions that last for a short period of time. Hibernation is a state of prolonged inactivity by sleeping during the winters. For example, animals like bears, squirrels, etc. Hence option c is incorrect.
Aestivation is a state of prolonged inactivity by sleeping during the summer. For example, animals including reptiles such as crocodiles, amphibians such as salamanders. Hence option d is incorrect.