Which of the following machines (devices) makes use of liquid as well as solid insulating materials?

A transformer uses both liquid and solid insulating materials for insulation and cooling.

Liquid Insulation:

• Transformer oil (mineral oil or synthetic oil) is used as a coolant and insulator.
• It helps to dissipate heat generated inside the transformer.

Solid Insulation:

• Paper, pressboard, and varnish are used to insulate windings and core components.

Additional Information 

• Synchronous Generator: Uses mainly air or hydrogen cooling with solid insulation but not liquid insulation.
• DC Generator: Uses solid insulation, but no liquid insulation.
• Induction Motor: Uses air or fan cooling, with solid insulation for windings, but no liquid insulation.