What would increase the force of gravity between two objects?
a. increase in mass
c. a and b
b. decrease in distance
d. increase in distance

By Newton's Law of Universal Gravitation.

F =  GMm/r²

Where F is Force of Gravitation, M = Mass of first object, m = mass of second object, r = distance of separation

From the formula, you can see that if the masses, M and m, increased, the value of F would definitely increase as well.

And if r increased the value of F would be reduced because you would be dividing by a bigger number, but when the value of r is decreased the value of F would be increased, because you would then be dividing by something smaller. Note the r is at the denominator of the formula.

So F would increase if there was increase in Masses and decrease in distance.

So the answer is C. a and b.