The correct answer is option 2: distance. A light year is a unit of measurement used in astronomy to measure large distances in space. It is the distance that light travels in one year in a vacuum. Since light travels at a speed of approximately 300,000 kilometers per second, in one year it can travel a very large distance. Therefore, a light year is used to express distances on a cosmic scale.

Option 1, time, is incorrect because a light year is not a measure of time but a measure of distance. It represents the distance that light can travel in a year.

Option 3, total amount of light falling on Earth in a year, is incorrect because a light year is a measure of distance, not the amount of light emitted or received.

Option 4, average intensity of light falling on Earth in a year, is also incorrect as a light year does not provide information about the intensity of light but rather the distance that light can travel in a year.

Therefore, the correct answer is option 2: distance, as a light year is a unit to measure the vast distances in space.