Explanation:

A light-year is a unit of length used to express astronomical distances. It is defined as the distance that light travels in a vacuum in one Julian year (365.25 days). Since the speed of light ($c$) is approximately $299,792,458 \text{ m/s}$, a light-year represents an immense distance, approximately $9.461 \times 10^{15} \text{ meters}$ or $9.461 \text{ trillion kilometers}$. Despite its name containing 'year', it is not a unit of time. Option A (Time) is incorrect as 'year' refers to the duration over which light travels, not the quantity being measured. Option C (Light) and Option D (Intensity of light) are incorrect as a light-year measures how far light travels, not light itself or its intensity. Thus, a light-year is fundamentally a unit of distance.