When you notice that someone has unusually blue eyes, you've noticed their
a. hybridization.
b. genotype.
c. phenotype.
d. allele.

Phenotype is being noticed when we notice unusually blue eyes.
Correct Option is C.

When stars first begin to fuse hydrogen to helium they lie on the zero age main sequence. the amount of time a star spends in the main sequence stage depends on its mass. eventually as stars age they evolve away from the main sequence to become red giants or supergiants