The correct answer is: B. Speed
A tachogenerator is a device that measures the speed of a rotating shaft or other rotating object. It does this by generating an electrical signal that is proportional to the speed of rotation. This signal can then be used to monitor the speed of the object, or to control the speed of a motor or other device.
Acceleration is the rate of change of speed. It is measured in units of meters per second squared (m/s^2). A tachogenerator cannot measure acceleration because it does not measure the rate of change of speed.
Displacement is the distance traveled by an object. It is measured in units of meters (m). A tachogenerator cannot measure displacement because it does not measure the distance traveled by an object.
Therefore, the only option that can be measured by the use of a tachogenerator is speed.