Animals that cannot regulate their body temperature internally and rely on environmental heat sources are known as:

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Question 14: Animals that cannot regulate their body temperature internally and rely on environmental heat sources are known as:

• Poikilothermic animals are those whose body temperature varies with the ambient temperature. They cannot maintain a constant internal body temperature and depend on external heat sources. This is also commonly referred to as being "cold-blooded" or ectothermic.
• Homothermic animals (also known as homeothermic or "warm-blooded" or endothermic) maintain a relatively constant internal body temperature, regardless of external temperature fluctuations, through internal metabolic processes.
• Polythermic is not a standard biological term for temperature regulation.

The description perfectly matches the definition of poikilothermic.

The final answer is $\text{A. Poikilothermic}$.