The correct answer is: D. Replace the poor soil
Black cotton soils are a type of clay soil that is found in tropical and subtropical regions. They are characterized by their high water content, which can make them difficult to work with and can cause problems for foundations.
The most suitable method to increase the bearing capacity of black cotton soils is to replace the poor soil with a more stable material. This can be done by excavating the existing soil and replacing it with sand, gravel, or crushed stone.
The other options are not as effective in increasing the bearing capacity of black cotton soils. Increasing the depth of the foundation will not improve the bearing capacity of the soil, as the soil will still be the same. Draining the soil will help to reduce the water content, but it will not completely eliminate it. Compacting the soil will help to improve its strength, but it will not be as effective as replacing the soil.
Therefore, the most suitable method to increase the bearing capacity of black cotton soils is to replace the poor soil with a more stable material.