The correct choice is (d) Mcr = βbZp fcr,b
To elaborate: The elastic critical moment is given by Mcr = √{[π^2EIy/ L^2MLT][ GIt + (π^2EIw/L^2LT)]} = βbZp fcr,b , Iy = moment of inertia about minor axis, Iw = warping constant, It = St. Venant’s constant, G = Shear modulus.