Final answer:
The distance of a galaxy is proportional to its radial velocity according to Hubble's Law, represented by the formula v = H x d, making the correct answer D) radial velocity.
Step-by-step explanation:
According to Hubble's Law, the distance of a galaxy is proportional to its radial velocity. Edwin Hubble found that galaxies appear to be moving away from the Earth, and the speed at which they do so (velocity of recession) increases with the distance from Earth. This relationship is quantified by the formula v = H x d, where v is the velocity at which the galaxy is moving away, H is the Hubble constant, and d is the distance of the galaxy from Earth. Therefore, the correct answer is D) radial velocity.