Stable oscillators are electronic circuits that generate continuous and periodic waveforms, such as sine waves or square waves, with a constant frequency. The output of a stable oscillator has a constant amplitude, which means that the peak or maximum value of the waveform remains the same over time. This is represented by option A, "Constant amplitude".

There are several reasons why the output of a stable oscillator has a constant amplitude:

1. Feedback Mechanism:
- Stable oscillators utilize a feedback mechanism to maintain oscillations. This feedback mechanism ensures that the output signal is continuously fed back to the input, which helps in sustaining the oscillations.
- The feedback mechanism is designed to provide the necessary gain and phase shift to maintain the desired frequency and amplitude of the oscillations. It ensures that the energy required for oscillations is continuously supplied to the circuit.

2. Active Devices:
- Stable oscillators typically use active devices, such as transistors or operational amplifiers, to amplify and shape the waveform.
- These active devices are designed to operate in their linear region, where they can provide consistent amplification without distortion. This allows the oscillator to generate a constant amplitude output.

3. Resonant Circuit:
- Stable oscillators often include a resonant circuit, which consists of capacitors and inductors, to determine the frequency of oscillation.
- The resonant circuit is designed to have a specific resonance frequency, at which the circuit exhibits maximum impedance. This ensures that the oscillator operates at a specific frequency with a constant amplitude output.

4. Control Mechanisms:
- Stable oscillators may include control mechanisms, such as voltage-controlled oscillators (VCOs) or temperature-compensated crystal oscillators (TCXOs), to maintain the desired frequency and amplitude.
- These control mechanisms continuously monitor the output signal and adjust the circuit parameters, such as the bias voltage or the resonant frequency, to compensate for any variations and ensure a constant output amplitude.

In conclusion, the output of a stable oscillator has a constant amplitude because of the feedback mechanism, the use of active devices, the presence of a resonant circuit, and the inclusion of control mechanisms. These factors work together to maintain the desired frequency and amplitude of the oscillations, resulting in a stable and consistent output waveform.