Explanation:

An overwhelming majority (99 percent) of animals and Nearly all plants cannot maintain a constant internal environment.

Their body temperature changes with the ambient temperature. In aquatic animals, the osmotic concentration of the body fluids changes with that of the ambient water osmotic concentration. These animals and plants are simply conformers. Considering the benefit of a constant internal environment to the organism, we must ask why these conformers had not evolved to become regulators.

Thermoregulation is energetically expensive for many organisms. This is particularly true for small animals like shrews and hummingbirds. Heat loss or heat gain is a function of surface area. Since small animals have a larger surface area relative to their volume, they tend to lose body heat very fast when it is cold outside; then they have to expend much energy to generate body heat through metabolism. This is the main reason why very small animals are rarely found in polar regions. During the course of evolution, the costs and benefits of maintaining a constant internal environment are taken into consideration. Some species have evolved the ability to regulate, but only over a limited range of environmental conditions, beyond which they simply conform. If the stressful external conditions are localized or remain only for a short duration, the organism has two other alternatives. Hence, option (a) is the correct answer.