As Blue whales migrate, they move between areas of high and low sunlight intensity. when they are in areas with more sunlight, their skin darkens in color. What could be the reason?

Due to more production of melanin.

Explanation:

Areas with more sunlight, the skin of the whale darkens in color because of more production of melanin. When the skin is exposed to the sun, it makes more melanin in order to protect the skin's lower layers from damage. When the skin becomes damaged due to sunburn, it produces more melanin. The extra melanin causes whales to become a darker color but it only occurs in those regions where sunlight intensity is higher.

It is A) The genes for melanin pigmentation remain turned on and prepare more melanin than usual to protect the skin from photo-damage when it is exposed to sunlight for long time.