The correct answer is B. 3 : 1.
The saturation gradient is the ratio of the hydraulic head at a point to the hydraulic head at a reference point. In ordinary loam soil, the saturation gradient is generally 3 : 1. This means that for every 3 units of hydraulic head at a point, there is 1 unit of hydraulic head at the reference point.
The saturation gradient is important because it determines the rate of flow of water in the soil. A higher saturation gradient means that water will flow more quickly through the soil. This can be important for irrigation and drainage applications.
The saturation gradient is also important for understanding the movement of contaminants in the soil. Contaminants will move more quickly through the soil when the saturation gradient is higher. This is why it is important to manage the saturation gradient in areas where there is a risk of contamination.
The other options are incorrect because they do not represent the typical saturation gradient in ordinary loam soil.