Final answer:
Galileo discovered that the Milky Way was composed of individual stars when he conducted the first telescopic survey of the Milky Way in 1610. This observation challenged previous beliefs about the Milky Way and contributed significantly to our understanding of the structure of the universe. Option a is correct.
Step-by-step explanation:
When Galileo first looked at the Milky Way with a telescope, he observed that the band of light was actually made of a) Individual stars. This discovery was made during his astronomical work that began late in 1609 and resulted in the first telescopic survey of the Milky Way in 1610.
Before his observations, nebulous regions in the sky were not clearly understood, but with his telescope, many such blurs resolved into numerous stars. The Milky Way, previously thought of as a continuous band of light, was revealed to be a dense collection of individual stars, now known to be part of the vast cosmic structure of our galaxy.
Today, we understand that the Milky Way is simply one galaxy among countless others in the universe, an insight that began with Galileo's pioneering observations. His work helped to deepen our understanding of the cosmos and laid the groundwork for future astronomical explorations.