Final answer:
The correct answer is D, which states space has continued to expand with clusters of galaxies moving away from each other, but with stars remaining in galaxies and galaxies remaining in clusters. This is supported by Edwin Hubble's discovery that the universe is expanding with galaxies showing a redshift in their spectra indicative of this expansion.
Step-by-step explanation:
The answer to the question lies in the understanding of the expanding universe postulated by the Big Bang theory. Option D is the most accurate. In summary, space continues to expand, and this expansion is evident in the increased separation of galactic clusters. Within these clusters, gravity holds galaxies together, and within galaxies, stars remain bound. It is essential to differentiate between the cosmic expansion of space itself and the motion of galaxies within that space.
Edwin Hubble's work in the 1920s showed that galaxies have a redshift in their hydrogen spectra, which is proportional to their distance from us. This implies that space itself is expanding, and there is no 'center' from which the expansion occurs. Instead, where you observe from seems like the center because everything moves away from that point in space. This phenomenon is not due to the Doppler shift but due to the expansion of space causing a cosmological redshift.
Ultimately, the concept is that space is stretching, leading to the increases in distance between large-scale structures like galaxies and galactic clusters. Galaxies within clusters and stars within galaxies remain bound by gravity and are not part of the large-scale universal expansion.