Datum
(reference point
from flight manual)
C.G. of mass
Front CG
limit
Rear CG
limit
Allowable CG range
Balance Arm
Elevator
Fin
Rudder
Stabilizer
Wing
Fuselage
The balance arm is the horizontal distance from the C.G. of a mass to the Datum line (Answer C)
The Datum is a fixed reference point defined in the aircraft flight manual (AFM/POH)