From this information, we can tell that the star cluster must be C. young, option C is correct
What happens when nearly all of stars fall along the main sequence?
An HR diagram is a scatter plot of stars that displays the correlation between the stars' effective temperatures or stellar classifications and their absolute magnitudes or luminosities.
The majority of the stars in the cluster are most likely in this stable stage of their existence since almost all of the stars in the cluster fall along the main sequence. Because stars spend a large chunk of their lives on the main sequence before evolving into other stages, such red giants or white dwarfs, this shows that the star cluster is young.