In a four-stroke petrol engine (actual cycle), the expansion stroke starts and ends approximately at:

Solution (By Examveda Team)

In actual (practical) petrol engines, combustion is initiated by advancing the ignition so that the pressure rise occurs around the time of TDC; as a result, the effective expansion (power) action on the piston begins a few degrees after TDC rather than exactly at TDC.

Therefore, the idea that the expansion starts about 30° after TDC is used to represent the practical situation where maximum useful pressure is developed slightly after TDC.

However, strictly speaking, the expansion ends near BDC. In practice the exhaust-valve timing causes gas release somewhat before or around BDC (typical exhaust valve events are tens of degrees before BDC), so any statement placing the end of expansion many degrees after BDC is imprecise.

Summary: Option C is the choice intended to represent the practical/start-after-TDC behaviour of the power stroke, but note that the exact end of expansion is actually around BDC (with valve events occurring slightly before BDC) rather than 50° after BDC — the key point is that the power stroke begins after TDC in real engines.