Concept:The expanding universe theory is based on the observation that galaxies are moving away from each other over time.
Explanation:Edwin Hubble, an American astronomer, made landmark observations in 1925.
He was the first to prove that the universe is expanding.
He discovered a direct relationship between the velocities of distant galaxies and their distances from Earth.
This relationship came to be known as Hubble's Law.
According to this law, galaxies that are farther away recede at faster speeds.
The rate of cosmic expansion is quantified by the Hubble Constant.
The Hubble Space Telescope was named in his honour.
The other astronomers in the options are not associated with this discovery.
Carl Edward Sagan was a science communicator, cosmologist, and astrobiologist.
Charles Messier was a French astronomer known for cataloguing nebulae and star clusters.
Tycho Brahe was a Danish astronomer famous for accurate and comprehensive astronomical observations.
Answer:The correct answer is Edwin Hubble, i.e., Option A.