The correct answer is A) Compressed air. Medium and slow speed diesels, which are commonly used in ships and boats, are typically started using compressed air. Compressed air is the primary starting medium for these types of diesel engines because it provides a reliable and efficient way to crank the engine and initiate the combustion process. The high pressure of the compressed air helps to overcome the initial resistance of the engine and get it running. The other options are incorrect because: B) Hydraulics are not commonly used for starting diesel engines, C) Ether is a flammable liquid that is not a suitable starting medium, and D) Electric starting motors are typically used for smaller, high-speed diesel engines, not the medium and slow speed diesels in question.