Which of the following cannot be used as an indication of wind direction?

The correct choice is swells, as they do not provide a reliable indication of wind direction. Unlike whitecaps, smoke, and waves, which all have characteristics that can directly reflect the current wind patterns, swells are more about the longer-term patterns of wind and are generated by distant weather systems.

Whitecaps can indicate that the wind is strong enough to create waves that break, and their orientation will usually align with the direction of the wind. Similarly, smoke can drift and spread in the direction of the wind, making it a useful visual aid in determining wind direction. Waves can also indicate wind direction by their crest and trough formations. However, swells are formed by winds that may have occurred far away from their present location, meaning they do not necessarily reflect the local wind direction but rather the cumulative effect of longer-range winds. Thus, swells are not suitable for discerning the current wind direction at a specific location.