The torque in AB is 800 N-m; in BC it is (800 - 300) = 500 N-m; in CD it is zero. The angle of twist cannot be greater than 1.5o, so
Let's see if this will satisfy the stress criterion...
Since the value of c will result in a stress less than 60 MPa, the diameter is 2c, or 42.1 mm. (Note that I doubled the unrounded value of c.)