The correct option is (c) √(fy /fcc)
The best I can explain: The value of non dimensional slenderness ratio λ in the equation of design compressive strength is given by λ = √(fy /fcc) , where fy is yield stress of material and fcc = (π^2E)/(KL/r)^2, where E is modulus of elasticity of material and KL/r is effective slenderness ratio i.e. ratio of effective length.