What term describes the use of a sub-audible tone transmitted along with normal voice audio to open the squelch of a receiver?

Answer: D. CTCSS

CTCSS (Continuous Tone-Coded Squelch System) sends a low, sub-audible tone along with your voice. A repeater or receiver set for that tone opens its squelch only when it hears it. DTMF uses audible touch-tone pairs instead.

Tip: CTCSS is the "PL tone" many repeaters require.