The correct answer is A. oven drying method.
The oven drying method is the most accurate method for determining the water content of soil. It is based on the principle that water evaporates at a constant rate when heated to 105°C. The soil is weighed before and after drying, and the difference in weight is the amount of water lost.
The sand bath method is less accurate than the oven drying method because it is difficult to control the temperature of the sand bath. The calcium carbide method is also less accurate because it is based on the assumption that all of the water in the soil is converted to calcium carbonate. The pycnometer method is the least accurate method because it is based on the assumption that the soil is a single phase.
The oven drying method is the most accurate method for determining the water content of soil because it is based on a sound scientific principle and it is easy to control the temperature. The other methods are less accurate because they are based on assumptions that may not be true in all cases.