The correct answer is: A. voltage booster.
A synchronous motor working at leading power factor can be used as a voltage booster because it can provide reactive power to the system. This reactive power can help to improve the power factor of the system, which can lead to lower losses and increased efficiency.
Option B is incorrect because a synchronous motor working at leading power factor cannot advance the phase of the voltage.
Option C is incorrect because a synchronous motor working at leading power factor does not generate noise.
Option D is incorrect because a synchronous motor working at leading power factor is not a mechanical synchronizer.