In case of biaxial stress (tensile), the maximum value of shear stress is not given by any of the options mentioned (a, b, c). The correct answer is option 'D' - None of these.

Explanation:
Biaxial stress refers to a situation where a material is subjected to two different normal stresses in perpendicular directions. In this case, the maximum value of shear stress does not depend on the difference or sum of the normal stresses.

The maximum value of shear stress in biaxial stress can be determined using the following equation:

τ_max = σ_max - σ_min / 2

where,
τ_max is the maximum shear stress
σ_max is the maximum normal stress
σ_min is the minimum normal stress

This equation shows that the maximum shear stress is equal to half the difference between the maximum and minimum normal stresses.

When the material is subjected to pure tensile stress in one direction, the normal stress in that direction is the maximum stress (σ_max), and the normal stress in the perpendicular direction is zero (σ_min = 0). In this case, the maximum shear stress is equal to half the maximum normal stress, which is the tensile stress.

However, in biaxial stress, both normal stresses are non-zero. The maximum normal stress may not be the tensile stress in one direction. Therefore, the maximum shear stress cannot be determined by the difference, sum, or half the sum of the normal stresses.

To determine the maximum shear stress in biaxial stress, the individual normal stresses need to be known. The maximum shear stress occurs on a plane inclined at 45 degrees to the principal planes of stress. The formula to determine the maximum shear stress on this plane is given by:

τ_max = (σ_1 - σ_2) / 2

where,
τ_max is the maximum shear stress
σ_1 is the principal stress with the largest magnitude
σ_2 is the principal stress with the smallest magnitude

Therefore, in the case of biaxial stress (tensile), the maximum value of shear stress is not given by any of the options mentioned (a, b, c). The correct answer is option 'D' - None of these.