maximum principle stress

Question

A shaft subject to torsion experiences a pure shear stress $\tau$on the surface. The maximum principal
stress on the surface which is at $45^0$to the axis will have a value

(a) $\tau$cos $45^0$                  (b) 2$\tau$cos$45^0$
(c) $\tau$$cos^2$$45^0$                    (d) 2$\tau$ sin$45^0$  cos $45^0$