Final answer:
The gravitational force between two objects is inversely proportional to the square of the distance between them. If the distance is doubled, the gravitational force is reduced to one fourth. Hence, the new force of attraction would be 4 units.
Step-by-step explanation:
The strength of the gravitational force between two objects depends on the mass of the objects and the distance between them. According to Newton's universal law of gravitation, the gravitational force is directly proportional to the product of their masses and inversely proportional to the square of the distance between them. So, when the distance between two objects is doubled, the gravitational force between them becomes inversely proportional to the square of this new distance, meaning it will be reduced to one fourth of the original force.
In the given scenario, if two objects attract each other with a gravitational force of 16 units and then the distance between them is doubled, the new force of attraction between the two objects will be 16 units divided by 4 (because of the squared relationship), resulting in a new force of 4 units. Therefore, the correct answer is a) 4 units.