Explanation: CTCSS (Continuous Tone-Coded Squelch System) functions by transmitting an inaudible, sub-audible analog tone along with the voice signal. A receiver equipped with CTCSS squelch is configured to recognize a specific tone frequency. The squelch circuit will only open its audio path (unmute the speaker) when it detects a received radio signal that contains this particular tone. This allows users to hear only traffic from others using the same tone, filtering out other signals on the same frequency. Therefore, **B) Tones** is correct because the presence of the correct sub-audible tone is the specific mechanism that causes the CTCSS squelch to open. **A) Noise** and **C) Absence of noise** are incorrect; while squelch generally deals with the presence or absence of a radio signal above the noise floor, CTCSS adds a layer of *tone detection* on top of this basic function. **D) Digital codes** refers to Digital Coded Squelch (DCS), a different system that uses digital bit patterns rather than continuous analog tones to achieve a similar selective squelch function.