If $\sin x =\large\frac{1}{\sqrt 3}$, then which of the following must be true ?

Question

$\begin{array}{1 1} (a)\; \cos^2x= \large\frac{3}{2} \\ (b)\; \cos^2x= \large\frac{2}{3} \\ (c)\; \cos^2x= \large\frac{1}{3} \\ (d)\; \cos^2x= 3\end{array} $