The correct answer is: C. The tangent of the angle of repose is equal to coefficient of friction.
The angle of repose is the maximum angle at which a granular material can be piled without slumping. The coefficient of friction is a measure of the resistance between two surfaces in contact. The tangent of an angle is the ratio of the opposite side to the adjacent side.
The tangent of the angle of repose is equal to the coefficient of friction because the angle of repose is the angle at which the force of friction between the granular material and the surface it is resting on is equal to the force of gravity pulling the material down.
The other options are not correct. Option A is incorrect because the tangent of an angle is not always equal to the coefficient of friction. Option B is incorrect because the angle of repose is not always equal to the angle of friction. Option D is incorrect because not all of the statements are true.