The blue colour of the deep sea is caused by:

Concept: The visible colours of sunlight are red, orange, yellow, green, blue, indigo, and violet (Arranged according to decreasing wavelengths). Water molecules, in general, absorb sunlight and scatter colours. Water absorbs colours with longer wavelengths more than colours with shorter wavelengths. When sunlight strikes water, the longer wavelengths of red, orange, yellow, and green are quickly absorbed by water molecules. Because those colours are absorbed, the light we see consists of blue and violet wavelengths. Water scatters these colours rather than absorbs them. As a result, we can see blue water in the deep sea.

Because the deep sea's blue colour is primarily due to scattering, the correct answer is option (C) Scattering of light.

Shorter wavelength colours disperse easily across water bodies, giving them a blue appearance.
This phenomenon is known as Rayleigh Scattering.