Maintaining constant internal conditions is a crucial aspect of an organism's survival and overall functioning. In this context, the term "conformers" refers to organisms that allow their internal conditions to change in response to external environmental changes. Let's break down the given options and understand why the correct answer is option 'D'.

a) Migration:
Migration refers to the movement of organisms from one region to another in response to changing environmental conditions. While migration can be a strategy employed by some organisms to maintain favorable conditions, it is not the main mechanism used by conformers to achieve internal stability. Conformers adjust their internal conditions without the need for migration.

b) Physiological means:
Physiological means refer to the various processes and mechanisms within an organism's body that contribute to maintaining internal conditions. While physiological means play a crucial role in achieving internal stability, it is not the defining characteristic of conformers. Conformers primarily rely on external factors to regulate their internal conditions.

c) Sub-optimal performance:
Sub-optimal performance refers to a state where an organism is operating below its optimal level due to environmental constraints. While conformers may experience sub-optimal performance when external conditions change drastically, it is not the primary mechanism by which they maintain constant internal conditions.

d) None of the above:
The correct answer is option 'D' because none of the given options accurately describe how conformers maintain constant internal conditions. Conformers exhibit a level of flexibility in allowing their internal conditions to change in response to the environment. This is in contrast to regulators, which actively maintain constant internal conditions through various physiological mechanisms.

Conformers rely on behavioral adaptations to adjust their internal conditions. These adaptations include seeking shade or sunlight, changing body posture, burrowing into the ground, or adjusting their activity levels. By altering their behavior in response to environmental changes, conformers can avoid extreme fluctuations in their internal conditions.

In conclusion, conformers maintain constant internal conditions primarily through behavioral adaptations rather than migration, physiological means, or sub-optimal performance. These adaptations allow them to adjust their internal conditions in response to external changes, ensuring their survival and functioning in varying environments.