Question

1. our sun will burn out when... the sun runs out of hydrogen to use as fuel. the amount of hydrogen and helium become equal. the sun shrinks too much to continue nuclear fusion. the sun runs out of helium as fuel.

Explanation:

The Sun generates energy through nuclear - fusion of hydrogen into helium in its core. When it runs out of hydrogen fuel, the fusion process will change, leading to the Sun's eventual burnout. The other options are incorrect as the balance of hydrogen and helium amounts is not the immediate cause, shrinking is a later consequence, and helium is a product of hydrogen fusion and not the initial fuel for the Sun's main - sequence lifetime.

Answer:

the Sun runs out of hydrogen to use as fuel.