The correct answer is (b) 400 metres.
Sunlight is made up of a spectrum of different wavelengths of light, from long red wavelengths to short violet wavelengths. When sunlight hits the ocean, some of the light is reflected off the surface, while some of the light is absorbed by the water. The amount of light that is absorbed depends on the wavelength of the light. Shorter wavelengths of light, such as violet and blue light, are absorbed more easily than longer wavelengths of light, such as red and orange light.
As sunlight travels through the water, it is absorbed by the water molecules. The deeper the sunlight travels, the more of the light is absorbed. This is why the ocean appears blue: the blue light is scattered in all directions by the water molecules, while the other colors of light are absorbed.
The sun’s rays can penetrate the clear ocean water to a maximum depth of about 400 meters. This is because the water molecules absorb most of the light at depths greater than 400 meters. At depths greater than 400 meters, the ocean is dark.
The other options are incorrect because they are not the maximum depth that the sun’s rays can penetrate the clear ocean water.