Correct answer is (a) (π/L){√[(EIyGIt) + (πE/L)^2IwIy]}
To elaborate: Elastic critical moment is given by Mcr = (π/L){√[(EIyGIt) + (πE/L)2IwIy]}, where EIy = flexural rigidity(minor axis), GIt = torsional rigidity, It = St.Venant torsion constant, Iw = St.Venant warping constant, L = unbraced length of beam subjected to constant moment in plane of web.