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.