Choice B is the correct answer. A perfect square trinomial fits the form $x^2 \pm 2bx + b^2$. The constant term must be the square of half the linear coefficient.

1. For $x^2 - 6x + 9$: Half of -6 is -3. $(-3)^2 = 9$. This matches.
2. It factors to $(x - 3)^2$.

Choice A is incorrect because $5^2 = 25 eq 20$. Choice C is incorrect because $2^2 = 4 eq 16$. Choice D is incorrect because the constant term in a perfect square trinomial must be positive (a real number squared is always positive).