The strength of the gravitational force between two objects depends on
a: their acceleration
b: their pressure
c: their velocity
d: their masses

Should be their masses. Because the strength of the gravitational force between two objects depends on two factors, mass and distance. the force of gravity the masses exert on each other. If one of the masses is doubled, the force of gravity between the objects is doubled. increases, the force of gravity decreases. 

D. The larger the object the greater the force. That's why we are pulled toward the ground because Earth is so large compared to us.

It may not seem like it, but we are pulling the Earth, too. But the force is so much smaller, that the net value pulls us toward Earth more so.

D)can be increased in a closed system