Temperature is considered as the most ecologically relevant environmental factor because it affects physiology, morphology, and geographical distribution of organisms. The correct answer is option 'D', which means all of these factors are influenced by temperature.

Physiology:
Temperature has a significant impact on the physiological processes of organisms. It affects enzyme activity, metabolic rate, and biochemical reactions within cells. Different organisms have different temperature optima, which is the range of temperature at which they function most effectively. Deviations from this optimum can lead to physiological stress or even death. For example, extreme heat can denature proteins and disrupt cellular functions, while extreme cold can slow down metabolic processes.

Morphology:
Temperature also influences the morphology or physical characteristics of organisms. It can affect body size, shape, coloration, and other external features. Many organisms exhibit adaptations to specific temperature ranges. For instance, polar bears have a thick layer of fat and dense fur to insulate themselves from the cold, while desert-dwelling animals often have long legs and large ears to dissipate heat more effectively.

Geographical Distribution:
Temperature plays a crucial role in determining the geographical distribution of organisms. Different species have specific temperature requirements and tolerances. This leads to the formation of temperature gradients across the Earth's surface, which in turn determines the distribution patterns of different organisms. For example, tropical rainforests are characterized by high temperatures, while polar regions have extremely low temperatures. Organisms have adapted to these specific temperature ranges and are found in regions where their physiological processes can function optimally.

Conclusion:
In conclusion, temperature is considered the most ecologically relevant environmental factor because it affects the physiology, morphology, and geographical distribution of organisms. It plays a vital role in determining an organism's ability to survive, reproduce, and thrive in its environment. Understanding the ecological impacts of temperature is crucial for studying and conserving biodiversity.