Shear stress and normal stress on any plane depend on how that plane is oriented. As you rotate the cutting plane through a stressed point, the normal stress keeps rising and falling, and it always hits its highest and lowest values exactly on the two planes where the shear stress is zero. That is basically the definition of a principal plane: it is the plane where the direct stress is stationary, either a maximum or a minimum, and this can only happen where there is no shear pulling sideways on it, because shear is what makes the stress keep changing as you rotate the plane. So the maximum and minimum normal stresses always sit on the principal planes, and the shear stress there is always zero, matching option (B).