The correct answer is $\boxed{\text{B) 3 hours}}$.
The inlet time is the time it takes for water to flow from the critical point to the drain. It can be calculated using the following equation:
$$\text{Inlet time} = \frac{\text{Distance}}{\text{Flow rate}}$$
The distance is the overland flow from the critical point to the drain, which is 8 km. The flow rate is the difference in level divided by the hydraulic conductivity of the soil. The hydraulic conductivity of the soil is a measure of how easily water can flow through the soil. It is typically measured in meters per day.
To calculate the inlet time, we need to know the hydraulic conductivity of the soil. However, this information is not given in the question. Therefore, we cannot calculate the inlet time exactly. However, we can estimate the inlet time by assuming a typical hydraulic conductivity for the soil. A typical hydraulic conductivity for a sandy soil is 10-6 m/day.
Using this value for the hydraulic conductivity, we can calculate the inlet time as follows:
$$\text{Inlet time} = \frac{8 \text{ km}}{10^{-6} \text{ m/day} \times 12.4 \text{ m}} = 3 \text{ hours}$$
Therefore, the inlet time is approximately 3 hours.