Which of the following could potentially cause a tractor to overheat?

Blocked coolant passages can significantly contribute to a tractor's overheating issue. Coolant is essential for dissipating heat generated by the engine during operation. When the passages that allow coolant to flow through the engine and radiator become blocked, the flow of coolant is restricted. This restriction can prevent the coolant from adequately absorbing heat, leading to higher engine temperatures.

In addition, blockages in the coolant passages can create localized hot spots within the engine, which may lead to further complications such as engine damage or failure if not addressed promptly. Ensuring that the coolant system is clean and free from obstructions is crucial for maintaining proper engine temperature and overall tractor performance.