If the normal stress is given, the shear stress on the plane is?

Question

On a plane resultant stress is inclined at an angle of 45⁰  to the plane. If the normal stress is

100 N/mm² the shear stress on the plane is

 (a) 71.5 N/mm²         (b) 100 N/mm²         (c) 86.6 N/mm²       (d) 120.8 N/mm²