Answer: The mass-luminosity relation applies to main-sequence stars (option a).
On the Hertzsprung-Russell (H-R) diagram, main-sequence stars are located along a diagonal line running from the upper left to the lower right, representing stars that are fusing hydrogen into helium in their cores. The mass-luminosity relation shows that a star's luminosity is directly related to its mass; more massive stars have higher luminosities.
The other luminosity classes, including supergiants, giants, and white dwarfs, have different characteristics and do not follow the mass-luminosity relation.
