The correct answer is C. Both front face and inner face.
A counterfort retaining wall is a type of retaining wall that is used to support a soil embankment. The wall consists of a stem, which is the vertical portion of the wall, and a series of counterforts, which are the horizontal projections that support the stem. The main reinforcement in the stem at mid span is provided on both the front face and the inner face of the wall. This is done to provide the wall with the necessary strength to resist the overturning moment caused by the soil pressure.
Option A is incorrect because the main reinforcement is not provided on the front face only. Option B is incorrect because the main reinforcement is not provided on the inner face only. Option D is incorrect because the main reinforcement is provided on both the front face and the inner face.