Correct answer: 0.5
Under pure torsion, the stress state at a point is pure shear with maximum shear stress $\tau_{max} = \tau$. The principal stresses are $\sigma_1 = \tau$ and $\sigma_2 = -\tau$, so the largest principal stress is $\tau$. Therefore, the ratio of maximum shear stress to largest principal stress is 0.5.