Final answer:
Gravity is an attractive force that exists between all massive objects as per Newton's universal law of gravitation. It not only keeps the planets in their orbits but also impacts the structure of space and time. The wide-reaching influence of gravity is evident in both terrestrial and cosmic phenomena.
Step-by-step explanation:
Gravity is the attractive force that operates between all masses. Its properties and implications were profoundly expanded upon by Sir Isaac Newton, who hypothesized through his universal law of gravitation that gravity extends beyond Earth and is the force that keeps planets in their orbits. The correct answer to the question regarding which objects gravity acts between is 'D. all massive objects.'
According to Newton's understanding, whenever there is mass, there is a gravitational attraction. This interaction is not merely localized to Earth and its immediate surroundings; it governs the motion of astronomical systems on a vast scale — from moons and planets to stars and galaxies. Moreover, Newton's law implies that gravitational force never completely vanishes; while it diminishes with distance, it persists endlessly.
One of the critical aspects of Newton's insight was realizing that the same force causing objects to fall to Earth is also responsible for celestial mechanics, like the Moon’s orbit. It also laid the groundwork for the understanding that gravity affects the very fabric of space and time itself, a concept that would be expanded upon by Einstein's theory of general relativity.