Explanation:

Definition of transparency:
Transparency refers to the property of a material that allows light to pass through it without scattering. Transparent objects are those that transmit light almost completely and can be seen clearly through.

Options given:
a) Clean water
b) Glass
c) Thin sheet of plastic
d) Muddy water

Analysis of each option:
a) Clean water: Clean water is a transparent object as it allows light to pass through it without scattering. When light passes through clean water, it is not obstructed and the object placed behind it can be clearly seen.

b) Glass: Glass is a transparent object as it allows light to pass through it without scattering. When light passes through glass, it is not obstructed and objects placed behind it can be clearly seen.

c) Thin sheet of plastic: A thin sheet of plastic can be transparent or translucent depending on its composition. If the plastic is clear and does not scatter light, it can be considered transparent. However, if the plastic is opaque or scatters light, it would not be transparent.

d) Muddy water: Muddy water is not a transparent object. It is a suspension of solid particles in water, which causes the water to become turbid and opaque. When light passes through muddy water, it is scattered by the suspended particles, making it difficult to see through the water clearly. Therefore, muddy water is not transparent.

Conclusion:
Out of the given options, the object that is not transparent is muddy water (option d). It does not allow light to pass through it clearly due to the suspended particles, making it opaque and turbid.