ANS:D - all of these.

1. Liquid state: In the liquid state, the soil behaves like a fluid, and its volume is primarily influenced by the amount of water present. A decrease in water content in the liquid state may not necessarily result in a significant reduction in volume because the soil particles are not densely packed and the void spaces between them are already relatively large.
2. Plastic state: In the plastic state, the soil is moldable and can be shaped without cracking. A decrease in water content in the plastic state leads to a reduction in volume because as water is removed, the soil particles come closer together, reducing the void spaces between them. This reduction in void spaces causes the soil mass to contract, resulting in a decrease in volume.
3. Semi-solid state: The semi-solid state is a transitional phase between the plastic and solid states. In this state, the soil has some degree of stiffness but is not as moldable as in the plastic state. Similar to the plastic state, a decrease in water content in the semi-solid state also results in a reduction in volume. As water is removed, the soil particles come closer together, causing the soil mass to contract and reducing its volume.

Therefore, while a decrease in water content primarily affects the volume of soil in the plastic state, it can also influence the volume of soil in the liquid state to some extent, and in the semi-solid state, it continues to contribute to volume reduction as the soil transitions towards a more solid-like state.