Final answer:
The washer in an air pump used with grease functions for compression, ensuring consistent application of the lubricant. It is not for lubrication itself, but an essential component for the air pump's proper operation.
Step-by-step explanation:
The washer of an air pump used with grease is primarily for compression. This component in an air pump system compresses the grease, allowing for consistent application of the lubricant where needed. While lubrication is closely associated with the substance being delivered, the washer itself is not for lubrication. While items like a) lubrication, d) protection of articular cartilage, and b) shock absorption might be correlated with grease or other components in a mechanical system, they are not the direct functions of a washer in an air pump.