$$\frac{\cos^2\theta}{\sin^2\theta} - \frac{1}{\sin^2\theta} = \frac{\cos^2\theta - 1}{\sin^2\theta} = \frac{-(1-\cos^2\theta)}{\sin^2\theta} = \frac{-\sin^2\theta}{\sin^2\theta} = -1$$
Answer: (D) −1
Use the identity $\sin^2\theta + \cos^2\theta = 1$, so $\cos^2\theta - 1 = -\sin^2\theta$. The expression simplifies neatly to −1. Source: Ch. 8, Section 8.4 Trigonometric Identities.