Answer: Functional distance
Explanation:
Functional distance is the distance that defines a spacing between people in terms of place of infrastructure that displays likeliness of people contacting and meeting each other.This distance plays an important role in relationship of people through meeting each other as per space.
According to the question, functional space is impacting friendship pattern in the case.As people who live near foot of stairs and mailboxes tend to have more friends because of nearness or less distances of houses but people living first floor have more distance among house so they have comparatively less friends.