The correct answer is: A. 95.0 MPa on net area.
The allowable tensile stress in structural mild steel plates for steel tank is assumed as 95.0 MPa on net area. This means that the maximum tensile stress that the plate can withstand without failing is 95.0 MPa. The net area is the area of the plate that is actually resisting the tensile force. This is calculated by subtracting the area of any holes or notches in the plate from the total area of the plate.
The other options are incorrect because they either overestimate or underestimate the allowable tensile stress. Option B, 105.5 MPa on net area, is too high. Option C, 105.5 MPa on gross area, is too low. Option D, 150.0 MPa on gross area, is much too high.
It is important to use the correct allowable tensile stress when designing a steel tank. If the stress is too high, the tank may fail. If the stress is too low, the tank may be unnecessarily heavy and expensive.