The correct answer is option C) shrinkage limit.

The shrinkage limit is the moisture content at which a soil reaches its minimum volume and further decrease in moisture content does not cause any significant change in volume. It is an important parameter used to characterize the behavior of soils.

Here is a detailed explanation of each term mentioned in the options:

a) Liquid limit: The liquid limit is the moisture content at which a soil transitions from a plastic state to a liquid state. At this moisture content, the soil behaves as a viscous fluid and has a tendency to flow. The liquid limit is determined by conducting a test called the Casagrande's liquid limit test.

b) Plastic limit: The plastic limit is the moisture content at which a soil transitions from a semi-solid state to a plastic state. At this moisture content, the soil can be molded or deformed without cracking. The plastic limit is determined by rolling a soil sample into a thread and observing the point at which it starts to crumble.

c) Shrinkage limit: The shrinkage limit is the moisture content at which a soil reaches its minimum volume and further decrease in moisture content does not cause any significant change in volume. It represents the lower limit of water content at which the soil can retain its shape. The shrinkage limit is determined by conducting a test called the ASTM D4943 test.

So, the correct answer is option C) shrinkage limit, as it specifically refers to the moisture content at which the soil volume becomes constant. The liquid limit and plastic limit are different parameters that characterize the behavior of soils at specific moisture contents.