The radiator is the core of the cooling system; its job is to pass heat from the hot circulating coolant into the air flowing through its fins, keeping the engine at a safe operating temperature.

As long as it works well, the coolant stays below its boiling point and remains a liquid throughout the system.

If the radiator cannot shed heat fast enough, the coolant temperature climbs until it reaches boiling point, at which stage it turns to steam and escapes, which is exactly the steam the driver sees.

Steam appearing after a long, hard drive therefore signals that the engine has overheated because the radiator is failing to remove heat effectively, perhaps due to blocked fins, low coolant, or a weak fan or pump.

The radiator has no role in cleaning fuel lines (option 1); that is the job of fuel filters, so steam says nothing about the fuel system.

It also does not maintain tyre pressure (option 3), and although overfilling (option 4) can push coolant out, a normal overflow is a steady liquid loss, not the boiling-off of steam that marks true overheating.

Hence, the answer is The radiator is failing to remove heat effectively.