The correct answer is option 'B' - Cruising.

Explanation:
- When a vehicle is idling, it means that the engine is running but the vehicle is not moving. In this state, the engine is still burning fuel to keep running, but there is no load on the engine. Therefore, the hydrocarbon discharge from the exhaust is higher compared to other states.
- When a vehicle is accelerating, the engine is working harder to increase the speed of the vehicle. This requires more fuel to be burned, resulting in higher hydrocarbon emissions from the exhaust.
- When a vehicle is decelerating, the engine is still burning fuel to keep running, but the vehicle is slowing down. In this state, the hydrocarbon discharge from the exhaust is higher compared to cruising, but lower compared to idling or accelerating.
- When a vehicle is cruising, it means that the engine is running and the vehicle is moving at a constant speed. At this state, the engine is under moderate load and is operating at an optimal efficiency. This means that the combustion process is more complete, resulting in lower hydrocarbon emissions from the exhaust.

Key Points:
- The discharge of hydrocarbons from petrol automobile exhaust is minimum when the vehicle is cruising.
- Idling and accelerating states result in higher hydrocarbon emissions.
- Cruising allows the engine to operate at an optimal efficiency, resulting in more complete combustion and lower hydrocarbon emissions.
- Decelerating state also leads to lower hydrocarbon emissions compared to idling or accelerating, but it is still higher than cruising.