Which characteristic of a star cluster is the best indicator of the cluster's age

a. the number of red giants in the cluster
b. the total number of stars of all types in the cluster
c. the faintest main sequence stars that can be seen in the cluster
d. the type of star that is leaving the main sequence

I know its not A, but I keep thinking that it should have something to do with luminosity or distance (or at least something related to HR diagrams)

Actually, I think it is A. It can be inferred that all the stars in a cluster were born at the same time (or close enough). More massive stars will have evolved off the main sequence into red giants. Because all the stars are the same distance means that the absolute magnitude of the red giants can be determined (and by inferrence its mass). With the properties known, the age of the red giants can be estimated. An older cluster will have more red giants among its members. That much is undeniable.

So what WAS the correct answer? Just curious.

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Yep, 'A' is the best answer.