The correct answer is A. decreases.
In a consolidated drained test, the soil sample is first consolidated under a confining pressure, and then sheared under drained conditions. This means that the water in the soil sample is allowed to flow out during the shearing process.
When a normally consolidated clay is sheared, the volume of the soil sample decreases. This is because the clay particles are rearranged during shearing, and the voids between the particles are reduced.
The other options are incorrect. Option B is incorrect because the volume of the soil sample decreases, not increases. Option C is incorrect because the volume of the soil sample changes, not remains unchanged. Option D is incorrect because the volume of the soil sample decreases throughout the shearing process, not first increases and then decreases.