Explanation: Crystal oscillators are valued in transmitters primarily for their exceptional **frequency stability**. A quartz crystal, when cut and mounted, resonates at a very precise frequency due to its piezoelectric properties. This natural resonance is highly stable and resistant to changes caused by temperature, voltage fluctuations, or aging, unlike simpler LC or RC oscillators. In amateur radio, maintaining a stable transmit frequency is crucial. It ensures clear communication, prevents drift into adjacent channels, and adheres to FCC regulations regarding spectral purity and band occupancy. An unstable transmitter would cause interference and make reliable contact difficult. * **A) No cross-modulation:** Cross-modulation is an issue typically associated with non-linear mixing of multiple signals, often in amplifiers or receivers, not a primary characteristic or benefit of a single-frequency crystal oscillator. * **B) Increase in gain:** An oscillator's primary function is to generate a signal, not to amplify it. Gain is provided by subsequent amplifier stages. * **D) Impedance gain and wider bandpass:** Crystal oscillators have a very high Q-factor, meaning they have an extremely *narrow* bandpass, which is precisely what contributes to their frequency stability, not a wider bandpass. Impedance matching is a separate circuit design consideration.