Answer: A. The distance light travels in a year.
Explanation: A lightyear is the distance light travels in side of a vacuum (space) in the period of one year. However, the really tricky part is, some years have 365 days in them, while some others have 366 (the leap year.) Currently, there seems to be no consensus as to the correct length of time. The reference found in Wikipedia was done at 365.25 days (translated to kilometers, that would be 9,460,730,472,580.8 km.)
