The correct answer is A. When external mechanical force is applied to it.
A piezoelectric crystal is a material that produces an electric charge when it is subjected to mechanical stress. This phenomenon is known as the piezoelectric effect. The reverse effect is also possible: when an electric voltage is applied to a piezoelectric crystal, it will deform. This property is used in many devices, such as microphones, loudspeakers, and sensors.
When an external mechanical force is applied to a piezoelectric crystal, the crystal deforms. This deformation causes the crystal to generate an electric charge. The amount of charge generated depends on the magnitude of the applied force and the size and shape of the crystal.
The piezoelectric effect is a reversible process. This means that when an electric voltage is applied to a piezoelectric crystal, the crystal will deform. The amount of deformation depends on the magnitude of the applied voltage and the size and shape of the crystal.
The piezoelectric effect is a useful property that has many applications. It is used in microphones, loudspeakers, and sensors. It is also used in some types of energy harvesting devices.
Options B, C, and D are incorrect because they do not describe the piezoelectric effect.