Which fuel, paired with liquid oxygen, powered the shuttle's main engines?

Liquid hydrogen, paired with liquid oxygen, powered the Shuttle’s main engines.

The RS-25 engines used cryogenic liquid hydrogen as fuel and liquid oxygen as the oxidizer. The external tank stored both propellants and fed them through the orbiter’s main propulsion system to the three engines during ascent.

Hydrogen was especially useful because it is extremely light and produces high exhaust velocity when burned with oxygen. The trade-off was difficult handling: liquid hydrogen must be kept near −253 °C, while liquid oxygen is stored near −183 °C. The tank required insulation and careful venting to manage these cryogenic fluids.

Several alternatives in the answer choices belong to other spacecraft systems or rocket designs. Hydrazine powered smaller attitude-control and auxiliary systems, while kerosene is used in many other launch vehicles. Methane was not the Shuttle main-engine fuel. The RS-25’s hydrogen-oxygen combination was central to the Shuttle’s high-performance ascent propulsion.