Correct Response: C. Edwin Hubble's observation of the redshift of galaxies proved that galaxies were moving away from Earth at an increasing speed directly proportional to their distance. This observation provides the basic evidence for an expanding universe. The distance between stars in the Milky Way (A) relates to the evolution of the Milky Way and local gravitational influences, not the expansion of the universe. The calculated distance to the cosmic horizon (B) is a theoretical boundary that does not provide direct evidence of the universe's expansion. The emptiness of most regions of space (D) is an observed property of the universe that is consistent with the idea of an expanding universe, but does not provide evidence for it.
