The governing principle is that a liquid boils when its vapour pressure becomes equal to the pressure acting on its surface, so the surrounding pressure directly sets the boiling temperature.

In an automobile cooling system the radiator pressure cap seals the circuit and lets the coolant build up above atmospheric pressure as the engine warms.

Because the coolant must now reach a higher vapour pressure to match this raised system pressure, it has to climb to a higher temperature before it can begin to boil.

This is why a pressurised system deliberately raises the boiling point: the engine can run hotter for better thermal efficiency without the coolant flashing into steam and causing loss of cooling.

The boiling point does not decrease, because higher pressure always suppresses boiling rather than encouraging it. It does not remain unchanged, since boiling temperature is genuinely pressure-dependent. It does not merely fluctuate, because the change is a steady, predictable rise with pressure.

Therefore raising system pressure has one definite effect on the coolant's boiling behaviour.

Hence, the answer is The boiling point of coolant increases.