Cryogenic engines are primarily used in rocket technology, which is why option 3 is the correct answer. These engines work on cryogenic fuels or substances that are required to be stored at very low temperatures. In rocket technology, they are particularly valuable because these fuels have a higher density and can produce a greater thrust, making rocket propulsion more effective and efficient.

Option 1 is incorrect as submarine propulsion primarily uses nuclear or diesel-electric engines. Cryogenic fuels, due to their extremely low temperature requirement, would be impractical and potentially dangerous for sub-marine use.

Option 2 is also incorrect because frost-free refrigerators function using heating elements to prevent the build-up of frost, not cryogenic engines.

Option 4, while involving extremely low temperatures similar to those in cryogenics, is also not the correct answer. Research in superconductivity generally involves materials and their ability to conduct electricity without resistance, rather than the use of engines. So, while the cryogenic temperatures might be used in the research, cryogenic engines would not be.