The correct answer is option 'C', the forced-circulation system, which is commonly employed in heavy trucks for engine cooling. This system is designed to efficiently regulate the temperature of the engine and prevent overheating. Let's explore the details of this cooling system.

Forced-circulation system:
The forced-circulation system is a popular choice for heavy trucks due to its effectiveness in cooling the engine. It utilizes a pump to circulate coolant throughout the engine, ensuring a constant flow of coolant and efficient heat transfer. Here's how the system works:

1. Coolant: The system uses a mixture of water and antifreeze, known as coolant, which has a higher boiling point and lower freezing point than water alone. This coolant is stored in a reservoir.

2. Pump: A water pump is responsible for circulating the coolant. It is typically driven by a belt connected to the engine. The pump forces the coolant from the reservoir into the engine block and cylinder head.

3. Engine Block: The coolant flows through channels within the engine block, which are strategically placed around the hottest areas of the engine. As the coolant passes through these channels, it absorbs heat from the engine components.

4. Cylinder Head: The coolant also circulates through the cylinder head, which houses the combustion chambers. This helps in cooling the cylinder head and preventing overheating.

5. Radiator: After absorbing heat from the engine, the hot coolant is then directed to the radiator. The radiator consists of many small tubes with fins attached to them, which increase the surface area for better heat dissipation.

6. Fan: To assist in cooling, a fan is mounted behind the radiator. It is either mechanically driven by the engine or electrically powered. The fan helps in drawing air through the radiator, facilitating heat exchange and cooling the coolant.

7. Cooling: As the hot coolant passes through the radiator, it releases heat to the surrounding air. This process cools down the coolant, which then returns to the engine to repeat the cycle.

The forced-circulation system is highly efficient in managing the engine temperature, especially in heavy trucks operating under demanding conditions. It ensures a continuous flow of coolant, preventing the engine from overheating and maintaining optimal performance.

In conclusion, heavy trucks commonly employ the forced-circulation system for engine cooling because of its effectiveness in regulating temperature and preventing overheating.