已知锐角 $\theta$ 满足 $\tan \theta=2$, 则 $\frac{1}{\cos ^{2} \theta}=(\quad)$
$\because \sin ^{2} \theta+\cos ^{2} \theta=1, \tan \theta=2$,

$$
\therefore \frac{1}{\cos ^{2} \theta}=\frac{\sin ^{2} \theta+\cos ^{2} \theta}{\cos ^{2} \theta}=\left(\frac{\sin \theta}{\cos \theta}\right)^{2}+1=\tan ^{2} \theta+1=2^{2}+1=4+1=5
$$

故选 $C$.