The major principal stress in the triaxial test refers to the highest stress acting on a soil specimen in the triaxial apparatus. It is an important parameter in soil mechanics and is determined through laboratory testing.

The triaxial test is a commonly used method to study the mechanical behavior of soils under different stress conditions. It involves placing a cylindrical soil specimen in a specially designed chamber and subjecting it to controlled axial and confining stresses. The axial stress is applied through a loading frame, while the confining stress is applied through a pressure cell surrounding the specimen.

During the triaxial test, the major principal stress is the stress acting along the axis of the specimen, which is also known as the axial stress. It is the primary stress that causes deformation and failure in the soil specimen. The major principal stress can be calculated by subtracting the cell pressure (confining stress) from the deviator stress.

- Deviator stress: The deviator stress is the difference between the axial stress and the cell pressure. It represents the stress that causes shear deformation in the soil specimen. In the triaxial test, the deviator stress is calculated by subtracting the cell pressure from the applied axial stress.

- Cell pressure: The cell pressure is the confining stress applied to the soil specimen through the pressure cell. It acts uniformly on all sides of the specimen and helps to simulate the stress conditions in the field. The cell pressure is typically applied hydrostatically and is equal in all directions.

- Major principal stress: The major principal stress is the highest stress acting on the soil specimen, which is the axial stress in the triaxial test. It is calculated by subtracting the cell pressure from the axial stress.

The correct answer to the question is option 'C', which states that the major principal stress is the deviator stress minus the cell pressure. This is because the major principal stress represents the axial stress, which is the difference between the deviator stress and the cell pressure.