Answer : 4. "distance"
Explanation :

Light-year is the unit of length to measure large distances (astronomical distances).

The total distance travelled by light in 1 year is called 1 light year.

1 light-year = 9.46 × 1012 km.

Explanation for the incorrect options

The second (sec) is the unit of time.

A kg is the unit of mass.

Therefore, the Light year is the unit of distance. So, option (D) is correct.