Final answer:
An open cluster on the main sequence represents the main sequence phase (c) in stellar evolution. It is a stage where stars are fusing hydrogen in their cores. More massive stars in a cluster advance more quickly beyond the main sequence, becoming red giants or supergiants.
Step-by-step explanation:
In stellar evolution, the phase represented by an open cluster on the main sequence is the main sequence phase (c). Open clusters are groups of stars that were formed around the same time and can be used to study different stages in the life of stars. Stars within these clusters that are still on the main sequence are undergoing hydrogen fusion in their cores. In contrast, more massive stars advance faster and may evolve off the main sequence to become red giants or supergiants. After the main sequence phase, a star like our Sun will expand and cool as it becomes a red giant, its core contracting while its outer layers expand. Ultimately, the star will shed its outer layers and the remaining core will cool and fade into a white dwarf.