The correct answer to the question is:
d. betweenness of points
Explanation:
1. When points A, B, and C are collinear, it means they lie on the same straight line.
2. The equation AB + BC = AC signifies that the distance from A to B added to the distance from B to C is equal to the distance from A to C.
3. This condition describes the concept of betweenness of points, where point B is positioned between points A and C on the line.
Therefore, the given statement defines the betweenness of points, which is the most accurate choice among the options provided.