Final answer:
The gravitational force between two objects when the distance is doubled results in a force one fourth of the original force. Hence, the new force of attraction between the two objects would be 4 units (16 units divided by 4).
Step-by-step explanation:
The strength of the gravitational force between two objects depends on mass and distance, according to Newton's law of universal gravitation. If two objects initially attract each other with a force of 16 units, and then the distance between the two objects is doubled, the new force of attraction can be calculated using the inverse square law. According to this law, the gravitational force is inversely proportional to the square of the distance between the objects.
When the distance is doubled, the denominator in the equation for the force of gravity becomes four times larger, since (2R)^2 = 4R^2. This implies that the gravitational force will be four times less powerful, so the new gravitational force will be 16 units divided by 4, resulting in a force of 4 units. Thus, option 1: 4 units, is the correct answer to this question.