The gravitational force between two bodies of mass M and m, separated by a distance r, has a magnitude F given by the universal law of gravitation,
F = G M m / r ²
where G is the universal gravitation constant (whose value is unimportant for this question).
If you double the distance r, then the new force has magnitude
G M m / (2r ) ² = 1/4 • G M m / r ²
i.e. 1/4 of the original force's magnitude. So the answer is D.
