Answer
$$\tan^2x+1=\sec^2x$$
$\text{C}$ is the answer here.
Work Step by Step
$$\tan^2 x+1$$
According to the Pythagorean Identities, we have
$$\tan^2\theta+1=\sec^2\theta$$
Therefore, $$\tan^2x+1=\sec^2x$$
We would pick $\text{C}$ as the answer here.