Answer: D
\[ \tan \theta = \frac{x}{y} \\ \Rightarrow \frac{\sin \theta}{\cos \theta} = \frac{x}{y} \\ \Rightarrow \frac{x \sin \theta}{y \cos \theta} = \frac{x^2}{y^2} \\ \Rightarrow \frac{x \sin \theta + y \cos \theta}{x \sin \theta - y \cos \theta} = \frac{x^2 + y^2}{x^2 - y^2} \\ \Rightarrow \frac{x \sin \theta - y \cos \theta}{x \sin \theta + y \cos \theta} = \frac{x^2 - y^2}{x^2 + y^2} \]
\[ \tan \theta = \frac{x}{y} \\ \Rightarrow \frac{\sin \theta}{\cos \theta} = \frac{x}{y} \\ \Rightarrow \frac{x \sin \theta}{y \cos \theta} = \frac{x^2}{y^2} \\ \Rightarrow \frac{x \sin \theta + y \cos \theta}{x \sin \theta - y \cos \theta} = \frac{x^2 + y^2}{x^2 - y^2} \\ \Rightarrow \frac{x \sin \theta - y \cos \theta}{x \sin \theta + y \cos \theta} = \frac{x^2 - y^2}{x^2 + y^2} \]