Explain why there must be at least two lines on any given plane.

The correct answer is:
there must be at least two lines on any plane because a plane is defined by 3 non-collinear points.

Explanation:
Since a plane is defined by 3 non-collinear points, we could have: a line and a point not on that line; two intersecting lines; two parallel lines; or simply 3 non-collinear points.
For 3 non-collinear points: If none of the 3 points are collinear, then we could have 3 lines, 1 going through each point. These lines may or may not intersect. If two of the 3 points are collinear, then we have a line through those 2 points as well as a line through the 3rd point.. Again, these lines may intersect, or they may be parallel.