Answer:
the set of point M, point N, and all the points between point M and point N
Explanation:
We assume you're interested in a description of line segment MN, the portion of the line containing M and N that has endpoints M and N. That description is found in "Answer:" above.
Other choices:
... all collinear points: describes line MN, not segment MN
... point M and point N: describes two points, not a line segment
... points the same distance from each: describes the line that is the perpendicular bisector of segment MN