Which component requires checking for clean coolant?

The coolant overflow reservoir is essential because it holds excess coolant that expands from the engine when it heats up and helps maintain the proper fluid level in the cooling system. Checking for clean coolant in the overflow reservoir is crucial because it can indicate the overall health of the cooling system. Contaminated coolant could lead to overheating or severe engine damage due to corrosion or blockage in the system. Ensuring that the coolant is clean helps in effective heat transfer and prevents the buildup of harmful deposits that could impair the system's function.

While other components like the power steering fluid reservoir, battery fluid, and oil reservoir are important to check for their specific functions, they do not relate directly to the coolant system and its performance in cooling the engine.