What factor may be degraded on wet ash?

The correct answer pertains to braking action, which can be significantly affected by wet ash on runways or taxiways. Wet ash creates a slippery surface that reduces the friction between an aircraft's tires and the runway. This diminished traction can lead to longer stopping distances, making it more challenging for pilots to control their aircraft during landing and taxiing.

In wet conditions, the presence of ash, which can absorb moisture, further exacerbates the slippery conditions. As a result, pilots and air traffic controllers must take extra precautions and may issue warnings regarding braking action under these circumstances to ensure safety during operations. The other factors like visibility, engine performance, or fuel efficiency may not be directly influenced by wet ash in the same way that braking action is, making it less critical in this specific context.