Choice A is the correct answer.

1. Factor: $(x - 1)(x - 3) \leq 0$.
2. Roots: 1 and 3.
3. Test: Between 1 and 3 (e.g., $x=2$), $(2-1)(2-3) = 1(-1) = -1$ (Negative).
4. Conclusion: The expression is $\leq 0$ between the roots, inclusive.

Choice B is incorrect because that is for $\geq 0$. Choice C is incorrect because it excludes the endpoints. Choice D is incorrect.