Put your number into the half-angle formula and simplify.
[tex]\displaystyle\sin{\frac{\theta}{2}}=\pm\sqrt{\frac{1-\cos{\theta}}{2}}=\sqrt{\frac{1-\frac{1}{3}}{2}}\\\\=\sqrt{\frac{1}{3}}=\sqrt{\frac{3}{9}}\\\\=\frac{\sqrt{3}}{3}[/tex]
The appropriate selection is ...
[tex]A)\quad\dfrac{\sqrt{3}}{3}[/tex]
