Answer:
C. lighter-than-air aircraft
Explanation:
A multirotor is a rotorcraft with more than two lift-generating rotors. An advantage of multirotor aircraft is the simpler rotor mechanics required for flight control. Multirotor drones are used by hobbyists so it could be recreational, but mostly it's used in radio control aircraft so not recreational. It's unmanned so it's not b. If not a or b it has to be c or d. Most of it's airborne time is it being lighter-than air. I'd choose c.
