Correct answer is (a) (π/L){√(EIyGIt)}
To elaborate: Long shallow girders have low warping stiffness or resistance. So, elastic critical moment for long shallow girders is given by (π/L){√(EIyGIt)}, where EIy = flexural rigidity(minor axis), GIt = torsional rigidity, L = unbraced length of beam subjected to constant moment in plane of web.