Correct answer is (a) Mdv = Md – β(Md – Mfd)
The best I can explain: The moment capacity of plastic or compact section for V > 0.6Vd is given by Mdv = Md – β(Md – Mfd), where Md = plastic design moment of whole section disregarding high shear force effect but considering web buckling effect, Mfd = plastic design strength of area of cross section excluding shear area, considering partial safety factor γm0, β is constant.