The correct answer is: B. falling head method
The falling head method is best suited for determination of permeability of coarse-grained soils because it is more accurate and reliable than the constant head method. The falling head method is based on the Darcy’s law, which states that the rate of flow of water through a porous medium is proportional to the hydraulic gradient. In the falling head method, the hydraulic gradient is determined by measuring the difference in head between two points in the soil and the time it takes for the water level to fall between these points. The permeability of the soil is then calculated using Darcy’s law.
The constant head method is also based on Darcy’s law, but it is less accurate and reliable than the falling head method. In the constant head method, the hydraulic gradient is determined by maintaining a constant head difference between two points in the soil. The rate of flow of water through the soil is then measured and used to calculate the permeability. However, the constant head method is not as accurate as the falling head method because it is difficult to maintain a constant head difference. This is because the water level in the soil can fluctuate due to changes in the water table or due to the movement of soil particles.
Therefore, the falling head method is the best method for determining the permeability of coarse-grained soils.