Animals that get most affected by the change in the environmental temperature are

The correct option is poikilothermic.

To understand why poikilothermic animals are the most affected by changes in environmental temperature, let us break down the terms and concepts:
1. Poikilothermic Animals (Ectotherms/Cold-blooded): These are organisms whose internal body temperature varies significantly and tends to match the temperature of their immediate environment. Since they lack internal physiological mechanisms to regulate and maintain a constant body temperature, any fluctuations in environmental temperature directly alter their body temperature, metabolic rates, and activity levels. Examples include amphibians, reptiles, and most fish.
2. Homeothermic Animals (Endotherms/Warm-blooded): These organisms maintain a stable internal body temperature regardless of external environmental fluctuations using metabolic heat production and physiological regulation (e.g., sweating, shivering). Birds and mammals are homeothermic and are therefore less directly affected by ambient temperature changes.
3. Aquatic and Desert Living Animals: These terms describe habitats rather than physiological mechanisms of temperature regulation. While desert or aquatic animals have specific adaptations to survive in their environments, their sensitivity to temperature changes ultimately depends on whether they are poikilothermic or homeothermic.

Therefore, animals that get most affected by changes in environmental temperature are poikilothermic animals because their survival and metabolic functions are directly dependent on external thermal conditions.