To have pressure wholly compressive under the base of a retaining wall of width b, the resultant of the weight of the wall and the pressure exerted by the retained, earth should have eccentricity not more than A. $$\frac{{\text{b}}}{3}$$ B. $$\frac{{\text{b}}}{4}$$ C. $$\frac{{\text{b}}}{5}$$ D. $$\frac{{\text{b}}}{6}$$

The correct answer is $\boxed{\frac{{\text{b}}}{6}}$.

The resultant of the weight of the wall and the pressure exerted by the retained earth should be within the middle third of the base of the retaining wall in order to have pressure wholly compressive under the base. This is because the maximum bending moment occurs when the resultant is located at the edge of the base. If the resultant is located within the middle third of the base, the bending moment will be less than the maximum bending moment and the pressure will be wholly compressive.

The eccentricity of the resultant is defined as the distance between the centroid of the base and the line of action of the resultant. The maximum eccentricity that can be tolerated for wholly compressive pressure is $\frac{{\text{b}}}{6}$. This is because the maximum bending moment occurs when the resultant is located at the edge of the base, which is $\frac{{\text{b}}}{2}$ from the centroid of the base. If the resultant is located within $\frac{{\text{b}}}{6}$ of the centroid of the base, the bending moment will be less than the maximum bending moment and the pressure will be wholly compressive.

The other options are incorrect because they are greater than $\frac{{\text{b}}}{6}$. If the eccentricity is greater than $\frac{{\text{b}}}{6}$, the pressure will not be wholly compressive and there will be a tensile stress at the bottom of the base.