Compare the everyday meanings of related words to fix the definition firmly.
- Subsonic: the prefix sub means below or under, so a subsonic plane flies below, slower than, the speed of sound.
- Sonic: exactly at the speed of sound.
- Supersonic: the prefix super means above or beyond, so a supersonic plane flies above, faster than, the speed of sound, typically labelled by its Mach number, where Mach 2 means twice the speed of sound.
- Hypersonic: an even more extreme case, five times the speed of sound or more, used for reentry vehicles and advanced missiles.
Since the question describes a plane specifically called supersonic, its defining feature is a cruising speed greater than the speed of sound in air, confirming option C and ruling out A, B and D.