Choice A is the correct answer. For one solution, $\Delta = 0$.

1. $b^2 - 4(1)(25) = 0$.
2. $b^2 - 100 = 0$.
3. $b^2 = 100$.
4. $b = \pm 10$.

Choice B is incorrect because it misses the negative case. Choice C is incorrect because it uses $\sqrt{25}$. Choice D is incorrect because it uses $\sqrt{25}$.