A solid circular beam with radius of $0.25$ m and length of $2$ m is subjected to a twisting moment of $20$ kNm about the z-axis at the free end, which is the only load acting as shown in the figure. The shear stress component $\tau_{xy}$ at point 'M' in the cross-section of the beam at a distance of $1$ m from the fixed end is

1. $0.0$ MPa
2. $0.51$ MPa
3. $0.815$ MPa
4. $2.0$ MPa