It is as a result of gravity. (D)
This is as stated by Newton's law of universal gravitation. That two objects in the universe attract one another with a force that is proportional to the product of their masses and inversely proportional to the square of the distance apart.
The constant of proportionality is the Universal Gravitational Constant.
G = 6.673 × 10⁻¹¹ Nm²kg⁻²