Step 1: Concept
Let $u = \sec x + \tan x$.
Step 2: Analysis
$du = (\sec x \tan x + \sec^2 x) dx = \sec x (\sec x + \tan x) dx$.
So, $\frac{du}{u} = \sec x dx$.
Step 3: Calculation
$\int \frac{1}{u^9} \cdot \frac{du}{u}$ is not correct here; rather, the substitution leads to a power rule application.
$\int u^{-10} du = \frac{u^{-9}}{-9} + C$.
Final Answer: (C)