Step 1: Concept
We use the method of substitution by identifying the derivative of the angle argument $x e^x$ in the numerator.
Step 2: Meaning
The derivative of $x e^x$ with respect to $x$ using the product rule is $e^x(1 + x)$. This matches the numerator exactly.
Step 3: Analysis
Let:
\[ u = x e^x \implies du = (1 \cdot e^x + x \cdot e^x) \, dx = e^x (1 + x) \, dx \]
Substitute these into the integral:
\[ I = \int \frac{du}{\cos^2 u} = \int \sec^2 u \, du \]
Using the standard integration formula $\int \sec^2 u \, du = \tan u + C$:
\[ I = \tan u + C = \tan(x e^x) + C \]
Step 4: Conclusion
The value of the indefinite integral is $\tan(x e^x) + C$.
Final Answer: (A)