The correct answer is D. Synchronous motor D.C. generator set.
A synchronous motor is an AC motor that runs at a constant speed, regardless of the load. This makes it ideal for use in a D.C. machines laboratory, where the speed of the motor must be precisely controlled. A D.C. generator is a device that converts mechanical energy into electrical energy. In a synchronous motor D.C. generator set, the synchronous motor is used to drive the D.C. generator. This provides a clean and reliable source of D.C. power for the laboratory.
The other options are not suitable for a D.C. machines laboratory. A rotary converter is a device that converts AC power to DC power. However, it is not as efficient as a synchronous motor D.C. generator set. A mercury arc rectifier is a device that converts AC power to DC power. However, it is not as reliable as a synchronous motor D.C. generator set. An induction motor is an AC motor that cannot be used to generate DC power.
In conclusion, the correct answer is D. Synchronous motor D.C. generator set.