Explanation:

When considering the discharge of hydrocarbons from petrol automobile exhaust, it is important to understand the different operating conditions of the vehicle. The options provided in the question are idling, cruising, acceleration, and deceleration.

Idling: When a vehicle is idling, it means the engine is running but the vehicle is not moving. This is typically the case when the vehicle is stationary, such as when waiting at a red light or stuck in traffic. During idling, the engine is running at a low RPM (revolutions per minute), resulting in incomplete combustion of the fuel-air mixture. This incomplete combustion leads to a higher discharge of hydrocarbons from the exhaust. Therefore, idling is not the condition in which the discharge of hydrocarbons is minimum.

Acceleration: Acceleration refers to the process of increasing the speed of the vehicle. During acceleration, the engine needs to deliver more power to overcome the resistance and increase the vehicle's speed. This requires a higher fuel-air mixture to be injected into the engine, resulting in a higher discharge of hydrocarbons from the exhaust. Therefore, acceleration is not the condition in which the discharge of hydrocarbons is minimum.

Deceleration: Deceleration, also known as braking or slowing down, occurs when the vehicle reduces its speed. During deceleration, the engine is typically at idle or running at a lower RPM. Although the engine is running at a lower RPM, the incomplete combustion and higher discharge of hydrocarbons still occur during deceleration. Therefore, deceleration is not the condition in which the discharge of hydrocarbons is minimum.

Cruising: Cruising refers to the steady-state operation of the vehicle at a constant speed, typically on a highway or open road. During cruising, the engine is running at a stable RPM, and the fuel-air mixture is optimized for efficient combustion. This results in a more complete combustion of the fuel, leading to a lower discharge of hydrocarbons from the exhaust. Therefore, cruising is the condition in which the discharge of hydrocarbons is minimum.

In conclusion, the correct answer is option 'B' - cruising. During cruising, the engine operates at a stable RPM and the fuel-air mixture is optimized for efficient combustion, resulting in a lower discharge of hydrocarbons from the exhaust.