Right answer is (b) Xfy / γm0
The explanation is: The design compressive strength in terms of stress reduction factor is given by fcd = Xfy / γm0 , where X = stress reduction factor for different buckling class, slenderness ratio and yield stress = 1/ [φ + (φ^2-λ^2)^0.5], fy is yield stress of material and γm0 is partial safety factor for material strength.