The stem of a cantilever retaining wall which retains earth level with top is 6 m. If the angle of repose and weight of the soil per cubic metre are 30° and 2000 kg respectively, the effective width of the stem at the bottom, is A. 51.5 B. 52.5 C. 53.5 D. 54.5

The correct answer is A. 51.5.

The effective width of the stem at the bottom is the width of the stem that is effective in resisting the lateral pressure of the soil. It is calculated by multiplying the height of the stem by the cosine of the angle of repose. In this case, the height of the stem is 6 m and the angle of repose is 30°, so the effective width of the stem at the bottom is $6 \cos 30^\circ = 51.5$ m.

Option B is incorrect because it is the width of the stem at the top. Option C is incorrect because it is the width of the stem at the middle. Option D is incorrect because it is the width of the stem at the bottom plus the width of the stem at the top.