Hint: Thermoregulation is the process by which animals maintain a perfect temperature for their survival. Cold-blooded animals can not maintain a constant temperature and therefore can not survive extreme conditions.
Pisces are cold-blooded animals as they can not control their body temperature according to the temperature of the surrounding.
Unlike warm-blooded animals, their body temperature fluctuates as they move in different surroundings with different temperatures. Thus the cold-blooded animals don’t have a constant body temperature and can not survive in extreme conditions. These mainly include reptiles, fishes, amphibians, insects, etc. They usually show either of the following 3 mechanisms for thermoregulation:
Additional Information: Warm-blooded animals include birds and mammals and are also known as homeothermic animals. Unlike cold-blooded animals, they can maintain a certain temperature even when the temperature of the surrounding changes. This allows them to warm up in cold areas and cool down in warmer areas, thus they can survive in extreme conditions. They have two mechanisms of thermoregulation which are endothermy and homeothermy.
So, the correct answer is "(B) Cold-blooded".
Note: In poikilothermy, the internal temperature of the animal might vary but the core temperature remains the same as the ambient temperature of the immediate environment but it changes drastically in the case of heterothermy. In ectotherms, the animal utilizes external means to control the temperature of their body. Oppositely, in endothermy, the animal uses internal factors like burning fat, panting, etc to maintain the body temperature. In homeothermy, animals maintain a constant temperature instead of varying external temperatures.