Final answer:
Stars with the largest radii on the Hertzsprung-Russell diagram are found in the red giants section, distinguished by their high luminosity and cool temperatures compared to main sequence stars.
Step-by-step explanation:
On a Hertzsprung-Russell diagram, you would find stars with the largest radii in the red giants region. These stars have evolved away from the main sequence and as they aged, have expanded to become giants or supergiants, possessing greatly increased radii. Red giants are characterized by their lower temperature and higher luminosity relative to most main sequence stars. In contrast, white dwarfs, which can also be found on the H-R diagram, are much smaller and denser, located at the lower left. The white dwarfs represent the evolutionary stage of stars that have shed their outer layers and have a much smaller radius than red giants.
While analyzing a star cluster's H-R diagram, to determine whether the stars above and to the right of the main sequence are red giants or protostars, one would consider their age and the cluster's overall age. Protostars are young and still contracting toward the main sequence, where they will spend the majority of their stellar lives. On the other hand, red giants are former main sequence stars that have expanded and cooled as they consume their nuclear fuel and evolve away from the main sequence.