Answer:
D. the sum of the distances to each focus equals a certain constant.
Explanation:
In mathematics, an elipse is defined as a curve which surrounds two focal points. The relation between the two focal points and a points on the curve is what defines an elipes, which is "the sum of the two distances to the focal points is a constant".
Therefore, the right answer to this question is D, because that's the defintion of an elipse.