What is the primary fuel mixture that needs to be ignited in a four-stroke engine?

In a four-stroke engine, the primary fuel mixture that needs to be ignited is air and fuel. This is because the four-stroke process consists of distinct strokes: intake, compression, power, and exhaust. During the intake stroke, the engine draws in a mixture of air and fuel into the combustion chamber.

Once this air-fuel mixture is compressed during the compression stroke, it reaches an optimal temperature and pressure, making it primed for ignition. The spark plug then ignites this mixture, causing a rapid expansion of gases that pushes the piston down during the power stroke, generating the engine's power.

The other options described do not represent the correct components necessary for ignition in a four-stroke engine. Water and fuel would not create a combustible mixture; air and oil are used in lubrication but not primarily for combustion, and fuel and lubricant would not lead to the ignition needed for engine operation. Thus, air and fuel is the correct pairing essential for initiating the combustion process in a four-stroke engine.