The gravitational force of two objects, by definition, is given by:

Where,
G: gravitational constant
m1: mass of object number 1.
m2: mass of object number 2.
d: distance between both objects.
Therefore, according to the given equation, a change that always results in an increase in gravitational force is:
Increase in the mass of the objects and decrease in the distance between them.
Answer:
A change that will always result in an increase in the gravitational force between two objects is:
Increase in the mass of the objects and decrease in the distance between them.