The correct answer is: C. Equal to its projection beyond wall base.
A lean concrete bed is a layer of concrete that is placed at the bottom of a wall footing. It is used to distribute the load of the wall evenly over the soil. The depth of the lean concrete bed should be equal to its projection beyond the wall base. This ensures that the load of the wall is evenly distributed and that the wall footing is stable.
Option A is incorrect because 10 cm is not a common depth for a lean concrete bed. Option B is incorrect because 15 cm is also not a common depth for a lean concrete bed. Option D is incorrect because the lean concrete bed should be equal to its projection beyond the wall base, not less than it.