The correct choice is (b) M = Zpγm0fy
To explain: The design capacity of channel/I-section purlin is given by Mdz = Zpzγm0/fy and Mdy = Zpyγm0/fy , Mdz and Mdy are design moment capacity about Z and Y axes, respectively, Zpz and Zpy are plastic section modulus about Z and Y axes, respectively and fy is yield stress of steel. For safety, design moment capacity should be always greater than or equal to factored bending moments.