The correct answer is: B. directly proportional to permeability of soil.
The time factor for a clay layer is a dimensionless parameter that is used to calculate the time it takes for water to flow through a clay layer. It is defined as the ratio of the drainage path to the permeability of the soil. The drainage path is the distance that water has to travel to reach the groundwater table, and the permeability of the soil is a measure of how easily water can flow through the soil. The time factor is directly proportional to the permeability of the soil, which means that the higher the permeability, the faster water will flow through the soil.
The other options are incorrect for the following reasons:
- Option A: The time factor is a dimensionless parameter, which means that it does not have any units.
- Option C: The time factor is inversely proportional to the drainage path, which means that the shorter the drainage path, the faster water will flow through the soil.
- Option D: The time factor is dependent on the thickness of the clay layer, which means that the thicker the clay layer, the longer it will take for water to flow through the soil.