Final answer:
When two points are used to define a plane, there is only one unique plane that can exist.
Step-by-step explanation:
When two points are used to define a plane, there is only one unique plane that can exist.
Imagine two points in space. We can draw an infinite number of lines passing through these two points. However, there is only one plane that can contain these two points and all the lines passing through them. This plane is determined by the two points and is unique.
Therefore, the correct answer is a) One unique plane.