Here's a problem to solve: The Sun is classified as a star. Which characteristic identifies the Sun as a star?

Among the 4 following options, which is the correct answer?

- A: The Sun spins on its axis.
 
- B: The Sun is located outside the Milky Way galaxy.
 
- C: The Sun is part of the Big Dipper.
 
- D: The Sun produces light and energy in a nuclear reaction.
 