Explanation: A grid-leak detector utilizes the grid-cathode junction of a vacuum tube as a rectifier. **A) Depends on the ability of the grid leak and capacitor to follow the average grid current variations which are directly proportioned to the modulation envelope:** This is accurate. The grid capacitor charges to the peak RF voltage, and the grid leak resistor provides a discharge path. The RC time constant of this circuit is critical; it must be short enough to allow the grid voltage to follow the relatively slow changes of the audio modulation envelope, but long enough to average out the much faster RF carrier cycles. This rectification and averaging process extracts the audio signal. **C) Very sensitive to weak signals:** Grid-leak detectors offer good sensitivity because the vacuum tube simultaneously performs detection (rectification) and amplification of the signal. The detected audio is then amplified by the tube itself, providing a stronger output for weak input signals compared to a simple diode detector. **B) Depends on second harmonic distortion:** While any non-linear device like a detector will generate harmonic distortion, the primary mechanism of AM detection for a grid-leak detector is rectification and averaging of the modulation envelope, not reliance on specific harmonic products. Therefore, since both A and C accurately describe key aspects of the grid-leak detector's operation, D is the correct choice.