The correct answer is (a) 1.5 τb sin2φ
The explanation: The value of Ψ in the fv for nominal shear strength according to tension field method is given by Ψ =1.5 τb sin2φ, where τb is buckling strength or shear stress corresponding to web buckling, φ is inclination of tension field which depends on depth of web and spacing of stiffeners.