A form of adaptive colouration that helps animals to remain unnoticed is

Answer Details

Countershading is a form of adaptive colouration in which an animal's dorsal (upper) surface is darker and its ventral (lower) surface is lighter. This colour gradient counteracts the natural shadowing effect caused by overhead light, making the animal appear flatter and less three-dimensional. The result is that the animal blends more effectively into its background and becomes harder for predators (or prey) to detect.

Countershading is widespread in nature. Many fish are dark on top and light on the belly, so they blend with the dark water below when viewed from above and with the bright sky when viewed from below. Deer, sharks, penguins, and many birds also display countershading.

The other options are not forms of colouration at all:

• Hibernation is a behavioural adaptation in which an animal enters a prolonged state of dormancy during cold seasons to conserve energy.
• Aestivation is a similar dormancy response to hot, dry conditions (e.g. lungfish burying in mud during drought).
• Migration is the seasonal movement of animals from one region to another in search of favourable conditions.

None of these three involves body colouration. The question specifically asks about adaptive colouration that helps animals remain unnoticed, which directly describes countershading.