Assertion: Cold blooded organisms show hibernation or aestivation.
Reason: Extremes of temperature cause activity of organisms.
A. Both assertion and reason are correct and reason is the correct explanation of assertion.
B. Both assertion and reason are correct but reason is not the correct explanation of assertion.
C. The assertion is correct but the reason is incorrect.
D. Both assertion and reason are incorrect.

Hint: Ectothermic and poikilothermic animals are both cold-blooded animals. Their bodies are unable to regulate their internal temperatures, hence their temperature is not consistent and changes depending on their surroundings. Cold-blooded animals seek warmth by lying perpendicular to the sun's rays, while they seek chill by lying parallel to the sun. Cold-blooded creatures can live on land or in water.

Compete answer:
Option A: Cold-blooded creatures cannot regulate their body temperature on their own, thus they must migrate or halt development for a period of time to avoid inactivity.
So, option A is correct.
Option B: Cold-blooded animals are those that are unable to regulate their internal body temperature in response to changes in the environment. They are unable to survive at severe temperatures. Reptiles, fish, and other cold-blooded animals are examples.
So, option B is incorrect.
Option C: Most cold-blooded species hibernate for months to get through the frigid winter, or have a short life span, as many insects do, and thus die.
So, option C is incorrect.
Option D: Cold-blooded animals are unable to maintain a steady internal temperature because they are unable to control their body temperature. Their body temperature tends to correspond to the temperature of the habitat in which they live.
So, option D is incorrect.

Hence, Option A is the correct answer.

Note:
Cold-blooded organisms characteristics:
In warm conditions, cold-blooded creatures are more active and may travel faster. When it is chilly, they are normally idle and relax. Cold-blooded species, such as insects and lizards, can be little or enormous, as crocodiles are. Because they can't manage their body temperature, cold-blooded animals can't keep a constant internal temperature. Their body temperature tends to match the temperature of the environment they live in. Their metabolism slows down when the temperature drops.