What is a ballast factor?

Pick one answer from A to D and explain why.

A$)$ The ratio of a lamps light output operated on a ballast to that when using a reference ballast

B$)$ The ratio of a starting current output to the operating current output for a lamp$-$ballast system

C$)$ The ratio of operating current output to input for a lamp$-$ballast system

D$)$ Another term for the efficacy of a lamp$-$ballast luminaire system