The correct answer is C. 9.
The total number of stress components at a point within a soil mass loaded at its boundary is 9. These stress components can be divided into three categories: normal stresses, shear stresses, and the stress tensor.
Normal stresses are the stresses that act perpendicular to the surface of the soil. There are three normal stresses at a point within a soil mass: the vertical stress, the horizontal stress, and the radial stress.
Shear stresses are the stresses that act parallel to the surface of the soil. There are three shear stresses at a point within a soil mass: the shear stress on the vertical plane, the shear stress on the horizontal plane, and the shear stress on the radial plane.
The stress tensor is a mathematical representation of the stress state at a point within a soil mass. It is a 3×3 matrix that contains the nine stress components.
The stress tensor can be used to calculate the principal stresses and the principal directions. The principal stresses are the three maximum and minimum stresses at a point within a soil mass. The principal directions are the three directions along which the principal stresses act.
The stress tensor can also be used to calculate the deviatoric stress and the spherical stress. The deviatoric stress is the part of the stress tensor that is responsible for the deformation of the soil. The spherical stress is the part of the stress tensor that is responsible for the change in volume of the soil.