An air-breather is installed on a fluid reservoir to:

An air-breather is specifically designed to vent the reservoir. Its primary function is to allow air to enter and escape the reservoir as fluids are drawn out or filled in. This prevents vacuum formation and maintains atmospheric pressure within the reservoir. By providing proper ventilation, the air-breather helps to avoid issues such as fluid cavitation and ensures that the fluid can move freely without creating blockages caused by pressure imbalances.

Allowing overflow, cooling the oil, or controlling air pollution does not accurately describe the essential purpose of an air-breather. Overflow systems typically involve separate mechanisms, cooling systems may require additional components for temperature regulation, and while air-breathers do allow air circulation, they are not primarily designed for environmental air quality control. Hence, the correct understanding of an air-breather's role in a fluid reservoir centers on its function to vent the system effectively.