Answer:
The correct answer is A. a very cold ecosystem, because they would lose too much body heat
Step-by-step explanation:
Unlike endotherms, ectotherms mainly depend on external heat sources, and their body temperature changes with the environmental temperature.
Their body temperature increases and decreases according to the external temperature. Although they might generate metabolic heat, they can not increase this heat production to maintain their internal temperature.
Ectotherms can regulate their temperature by looking for areas that satisfy their needs, like direct sun, or some shadow. But they always need sources that provide them with heat.
It is very unlikely that ectotherms live in a very cold ecosystem because, as they depend on warm places or areas, they would lose too much body heat. Also, as they can only produce metabolic heat, it would be impossible or them to gain it again.