Question

Which mathematical relationship best describes a graph of gravitational field strength versus distance? 

(one of the 3 pictures attached or none of the above?)

Answer 1

gravitationnal field between two objets = k.m1.m2/d²
none of the above
the graph is something like that

Answer 2

The gravitational force between two masses is proportional to

(mass-1) x (mass-2) / (the distance between them)-squared

Do you see (distance-squared) in the denominator there ?

That steers you straight to the function [ y proportional to 1/x² ] ... in the middle.