Under what condition would a small unmanned aircraft not have to be registered before it is operated in the united states?

a. when the aircraft has a takeoff weight that is more than .55 pounds, but less than 55 pounds, not including fuel and necessary attachments
b. when the aircraft weighs less than .55 pounds on takeoff, including everything that is on-board or attached to the aircraft
c. all small unmanned aircraft need to be registered regardless of the weight of the aircraft before, during, or after the flight