Final answer:
A star spends about 90% of its lifetime on the main sequence of the H-R diagram, where it fuses hydrogen into helium. The correct answer is d) The entire lifetime.
Step-by-step explanation:
Over the course of a star's lifetime, it undergoes various changes that alter its position on the Hertzsprung-Russell (H-R) diagram, which plots stellar luminosity against surface temperature. During a star's life, it spends about 90% of its lifetime in a phase where it would be plotted along the main sequence of the H-R diagram.
This is due to the fact that during the main sequence phase, stars are fusing hydrogen in their cores into helium, which is a stable and long-lasting source of energy. As a result, a star remains on the main sequence for the majority of its existence before moving off to become a giant, supergiant, or white dwarf at the end of its life.
The Hertzsprung-Russell diagram (H-R diagram) is a plot of stellar luminosity against surface temperature. Most stars lie on the main sequence, which extends diagonally across the H-R diagram from high temperature and high luminosity to low temperature and low luminosity.
High-mass stars emit more energy and are hotter than low-mass stars on the main sequence. Main-sequence stars derive their energy from the fusion of protons to helium. About 90% of the stars lie on the main sequence.