#### Answer

$$\cos^2 x=\frac{1}{\sec^2 x}$$
$\text{B}$ is the answer.

#### Work Step by Step

$$A=\cos^2 x$$
According to this Reciprocal Identity:
$$\sec\theta=\frac{1}{\cos\theta}$$
$$\cos\theta=\frac{1}{\sec\theta}$$
Therefore, $A$ can be rewritten as follows:
$$A=\frac{1}{\sec^2 x}$$
$\text{B}$ is the answer.