A thin layer of water is transparent but a very thick layer of water is:
C.)Most transparent
D.)Completely transparent

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Hint: Remember that thicker the medium, the more likely it is for light entering it to undergo attenuating processes that reduce light propagation through the medium. To this end, you could also visualize this difference by thinking of when you drop a bit of water on your desk in contrast to looking down the deep end of a swimming pool.

Complete step by step answer:
Let us try and understand what the terms “transparent” and “translucent” mean in this context.
When light rays encounter a material medium they can undergo either reflection, absorption or transmission, and the extent of this interaction depends on the wavelength of incident light and the nature of the material medium.
Transparency is a physical attribute that a material medium lends to its structure where it allows light rays to pass through it in such a way that any object situated beyond or behind it can be distinctly seen. This happens when there is a nearly negligible scattering of light that is a result of minimum reflection and maximum transmission of inbound light rays.
Translucency is also a physical attribute that a material medium lends to its structure where it allows light to pass through but some of the emergent light may bet attenuated via reflection, absorption or scattering.
Layers of water act as refractive media. The deeper the water is, the more likely it is for the incoming light ray to have parts of it scattered, reflected, refracted or absorbed, all of which are light attenuation processes.
A thick layer of water has more strata of molecular water films than a thin layer of water. This gives more opportunity to the medium to deteriorate parts of the incident light to collateral processes that diminish the intensity and laterally shift the direction of the emergent light, which in turn takes away from the “transparency” of the medium and partially obstructs its transmission.
Hence, the correct answer is option A.

Additional Information
A good way to quantitatively analyse the response of media to the transmission of light through them would be to approach it from the perspective of Snell’s Law, which relates transparency and translucency of the media with respect to the angle of incidence of the incoming beam and its angle of refraction as it passes through the medium. This in turn determines the reflection and transmission probabilities which can be used to deduce the extent of opacity of the medium to light.

It is crucial to remember the differences between transparent (allows all the incident light to pass through, owing to lack of reflective or absorptive processes), translucent (attenuates part of the incident light but allows most of the light to pass through, owing to reflective, refractive and absorptive processes) and opaque (does not allow light to pass through, owing to large reflective processes) media, and how their behaviour is sometimes subject to change depending on the incidence angle of the beam.