Explanation: The piezoelectric effect describes the phenomenon where certain crystalline materials, like quartz, exhibit a reciprocal relationship between mechanical stress and electrical voltage. When a voltage is applied across a piezoelectric crystal, it mechanically deforms or vibrates. Conversely, when mechanical pressure or vibration is applied to the crystal, it generates an electrical voltage. This principle is fundamental in amateur radio because crystal oscillators rely on this effect to produce stable, precise frequencies. Applying an AC voltage at the crystal's resonant frequency causes it to vibrate, and this mechanical vibration, in turn, helps sustain the electrical oscillation, providing the stable reference needed for transmitters and receivers. B) Mechanical deformation by a magnetic field is not the piezoelectric effect. Magnetic fields interact with magnetic materials or moving charges. C) Generating electrical energy from light describes the photovoltaic effect (solar cells), not piezoelectricity. D) Reversed conduction states in a P-N junction due to light relates to photodiodes or solar cell operation, not the piezoelectric effect.