Final answer:
The Himalayan rabbit's fur turns black in cooler temperatures due to a temperature-sensitive gene that affects pigment production. This genetically determined trait helps the rabbit adapt to cold environments. The correct option for why the rabbit's extremities turn black is related to the temperature sensitivity of the pigment-producing gene.
Step-by-step explanation:
The phenomenon in question involves a Himalayan rabbit turning black in response to certain environmental conditions. This change in coloration is due to a temperature-sensitive gene product that only produces pigment in cooler areas of the rabbit's body. Specifically, the protein product does not fold correctly at higher temperatures, but does so at temperatures below 3.5°C, leading to the production of pigment in those cooler areas, thus causing the rabbit's fur to turn black.
Regarding Himalayan rabbits and other animals adapted to cold environments, it is known that mutations that cause changes in coloration due to environmental temperatures can be beneficial, and therefore, persist within a population. These temperature-sensitive proteins have similar impacts on the color of Siamese cats, arctic foxes, and other animals that change color with the seasons due to temperature changes, with fur being darker in colder temperatures and lighter in warmer temperatures.
Based on the information provided, the most likely cause of the black coloration in the colder parts of the rabbit's body is associated with the rabbit's adaptation to cold temperatures, and not by lack of oxygen, sun exposure, hibernation, or illness. Therefore, the correct option in the final answer directly relates to the genetic and physiological response to cold temperature.