An oil cooler is a heat exchanger in which hot engine oil gives up its heat to the engine cooling water; the oil is cooled only because the water flowing through the cooler is at a lower temperature than the oil.

The rate of heat transfer follows the temperature difference between the two fluids — the bigger the gap between hot oil and cooler water, the more heat leaves the oil for a given oil flow and pressure.

If the cooling water temperature unexpectedly rises above normal, that temperature difference shrinks, so for the same oil flow and oil pressure less heat can be pulled out of the oil.

With less heat being removed, the oil's own temperature climbs even though its pressure and flow are unchanged, which is precisely why the cooler becomes less effective.

The option claiming faster cooling is wrong because a hotter coolant reduces, not raises, the heat-exchange driving force; a smaller temperature gap means slower heat removal.

The “compensates automatically” and “increasing water pressure” options are also wrong, since a plain oil cooler has no self-regulating action and raising water pressure does not lower the water temperature that actually governs cooling.

Hence, the answer is The oil cooler becomes less effective, resulting in higher oil temperature.