9. If the graph of the function $y=\sqrt{3} \cos x-\sin x$ is shifted to the right by $m(m>0)$ units, and the resulting graph is symmetric about the $y$-axis, then the minimum value of $m$ is ( ).
(A) $\frac{\pi}{3}$
(B) $\frac{2 \pi}{3}$
(C) $\frac{\pi}{6}$
(D) $\frac{5 \pi}{6}$
9. C.

Notice that
$$
y=\sqrt{3} \cos x-\sin x=2 \cos \left(x+\frac{\pi}{6}\right),
$$

the equation of the axis of symmetry is $x=k \pi-\frac{\pi}{6}(k \in \mathbf{Z})$. Therefore, $m_{\min }=\frac{\pi}{6}$.