S, u , n are collinear. select the point that satisfies the definition of betweenness if nu + us = ns .

Point U is the point that satisfies the definition of betweenness.

We are given that the three points S,U and N are collinear (i.e. they lie on a line).

By definition of betweenness we mean, a point B is between two other points A and C if all three points are collinear and AB +BC = AC.

Hence, from the given property:

NU + US = NS  we have:

U is the mid point of the line joining the points N and S.

Hence, the point that satisfies the definition of betweenness is:

Point U.