Final answer:
The Sun is currently in the main-sequence phase of its evolutionary life, deriving its energy from the conversion of hydrogen to helium through nuclear fusion in its core.
Step-by-step explanation:
The Sun is currently in the main-sequence phase of its evolutionary life, making it option a. main sequence—middle age. The main-sequence phase is the stage where a star, like the Sun, derives its energy from the conversion of hydrogen to helium through nuclear fusion in its core. This process allows the star to maintain its equilibrium for a long time.