Doubly-excited magnetic systems refer to systems that have two independent sources of excitation, resulting in a magnetic field that is produced by two separate windings. These windings are often referred to as the field winding and the armature winding. In this case, all of the mentioned options, namely synchronous machines, loudspeakers and tachometers, and D.C shunt machines, are examples of doubly-excited magnetic systems.

Synchronous Machines:
Synchronous machines are AC machines that can operate as generators or motors. They have a field winding placed on the rotor and an armature winding on the stator. The field winding is excited by a DC source, while the armature winding carries the AC current. This dual excitation creates a magnetic field that interacts with the armature winding, resulting in the generation or conversion of electrical power.

Loudspeakers and Tachometers:
Loudspeakers and tachometers are examples of electromagnetic devices that utilize doubly-excited magnetic systems. In a loudspeaker, the field winding is represented by a permanent magnet, while the armature winding is created by a coil of wire attached to the diaphragm. The interaction between the magnetic field produced by the permanent magnet and the current flowing through the coil generates sound.

Similarly, in a tachometer, the field winding is represented by a permanent magnet, while the armature winding is created by a coil of wire. The rotation of the armature winding generates a voltage that is proportional to the speed of rotation, allowing the tachometer to measure the rotational speed of a machine.

D.C Shunt Machines:
D.C shunt machines are another example of doubly-excited magnetic systems. They have a field winding connected in parallel with the armature winding. The field winding is excited by a separate DC source, while the armature winding carries the load current. The interaction between the magnetic field produced by the field winding and the current flowing through the armature winding enables the machine to generate or convert electrical power.

In conclusion, all of the mentioned options, namely synchronous machines, loudspeakers and tachometers, and D.C shunt machines, are examples of doubly-excited magnetic systems. They all utilize two separate windings, namely the field winding and the armature winding, which interact to produce the desired electromagnetic effects.