the distance light travels in one year. the of time it takes light to travel from Earth to the Sun. the amount of light required on Earth in one year. the distance between two sources of light.
I gave my students this multiple choice question: A "light year" refers to

Only one answer is correct among these 4 choices:
- the distance light travels in one year.
- the of time it takes light to travel from Earth to the Sun.
- the amount of light required on Earth in one year.
- the distance between two sources of light.

Could you tell me which one is correct?

the distance light travels in one year.