The correct answer is option 3: "Liquid hydrogen as fuel and liquid oxygen as oxidizer."

In cryogenic engines, which are commonly used in space exploration, the fuel and oxidizer play essential roles in the combustion process. The fuel is the substance that undergoes combustion and provides the energy for propulsion. The oxidizer, on the other hand, is needed to sustain the combustion by supplying oxygen or other oxidizing agents.

Liquid hydrogen is commonly used as fuel in cryogenic engines due to its high energy content and low molecular weight, which makes it efficient for achieving high thrust levels. It is also capable of burning cleanly and producing water vapor as a byproduct, making it environmentally friendly.

Liquid oxygen, on the other hand, is used as the oxidizer in cryogenic engines. It provides the necessary oxygen for the combustion of the fuel. Liquid oxygen is advantageous because it is highly reactive and supports combustion at very low temperatures. Its properties make it suitable for use in cryogenic engines, where extreme temperatures are encountered.

Therefore, the most widely used cryogenic engines employ liquid hydrogen as fuel and liquid oxygen as the oxidizer to achieve optimal combustion and propel spacecraft effectively.