In statement 1. it refers to a permanent magnet that is coupled to the engine and as it rotates within a 3 phase stator the resulting 3 phase ac supply drives a 3 phase ac synchronous motor that rotates a drag cup around a permanent magnet. The magnet rotates against a hairspring and indicates the rpm.

In statement 2. it refers to a device that converts ac signals into DC that drives a dc motor. The output is then compared to a dc reference output and the difference indicates the rpm.

In statement 3. it refers to a device that is used on some jet engine aircraft to sense the shaft speed of the LP compressor during the engine start up cycle. It uses a single AC phase of a normal tacho generator to show that the shaft is rotating and is automatically isolated after the atart cycle has been sucessfully completed.

In statement 4. it refers to a direct drive 3 phase ac generator that is operated in conjunction with a 3 phase ac synchronous motor to indicate the rpm.

The correct answer is therefore 1, 2, 3, and 4 as given.