The correct answer is A. Mechanical displacement into a change of resistance.
A strain gauge is a device used to measure the strain on an object. Strain is the deformation of an object due to an applied force. Strain gauges are typically made of a thin metal foil that is bonded to the surface of the object being measured. When the object is deformed, the strain gauge also deforms, and this deformation causes a change in the resistance of the strain gauge. This change in resistance can be measured and used to calculate the strain on the object.
Strain gauges are used in a variety of applications, including structural analysis, machine condition monitoring, and force measurement.
Option B is incorrect because a strain gauge does not measure pressure. Pressure is a force per unit area, and a strain gauge measures the deformation of an object.
Option C is incorrect because a strain gauge does not measure force. Force is a push or pull on an object, and a strain gauge measures the deformation of an object.
Option D is incorrect because a strain gauge does not measure displacement. Displacement is the change in position of an object, and a strain gauge measures the deformation of an object.