Final answer:
Thrust in jet-propelled vehicles is produced by fuel combustion. It propels the vehicle forward due to Newton's third law, where expelling exhaust gases backward results in a reaction force that moves the vehicle forward.
Step-by-step explanation:
In a jet-propelled vehicle, the thrust that propels it forward is produced by fuel combustion. The principle of thrust is a reaction force that pushes a body forward in response to a backward force. In the case of jet engines and rockets, the vehicle moves forward by expelling exhaust gases backward at high velocity. The force that discharges the exhaust gases from the rear is accompanied by an equal and opposite reaction force that pushes the vehicle forward, according to Newton's third law of motion.
Rockets, in particular, do not rely on pushing against the ground or the air behind them; they operate more effectively in a vacuum, which allows for the unimpeded expulsion of exhaust gases. This means the correct answer is (a) Fuel combustion, as that is the source of the force that emits the gas backward resulting in thrust.