What Is "Functional Distance" to a Social Psychologist

What is "functional distance" to a social psychologist?

A) It is the distance between you and your best friend.
B) It is an aspect of architectural design that determines what people you see most often.
C) It is a measure of how close you feel to your family and friends.
D) It is the distance between your home and your job. It defines how many people you are likely to see on your way to work.