Which device can be used to enter the operating frequency on a modern transceiver?

The keypad or VFO knob is indeed the primary method used to enter or select the operating frequency on a modern transceiver.

Keypads typically allow the operator to input specific frequencies directly, making it straightforward to dial in the desired frequency for communication. The VFO (Variable Frequency Oscillator) knob is used for tuning, enabling the user to adjust the frequency smoothly and continuously. This flexibility is crucial for making fine adjustments and finding clear signals or specific stations.

The other devices mentioned, such as the CTCSS or DTMF encoder, serve different functions. CTCSS (Continuous Tone-Coded Squelch System) is mainly used for privacy and to prevent unwanted audio interference by only allowing signals with the correct tone to be heard. DTMF (Dual-Tone Multi-Frequency) encoders are typically used for touch-tone dialing, often in repeater systems, rather than for frequency selection. The Automatic Frequency Control (AFC) functions to stabilize and maintain the operating frequency automatically, but it does not provide a means to select or enter a new operating frequency.

Thus, the correct answer focuses on the primary, straightforward methods available for frequency selection on modern transceivers, which are through the keypad and the VFO knob.