The correct answer is (b) (1-βf)βf Iy h^2f
The explanation: Warping constant in elastic critical moment is given by Iw = (1-βf)βf Iy h^2f , where βf is ratio of moment of inertia of compression flange to sum of moments of inertia of compression and tension flanges, Iy = moment of inertia about minor axis, hf = centre-to-centre distance between flanges.