What weather phenomenon is guaranteed to occur when flying across a front?

When flying across a front, a change in wind direction is indeed one of the most consistent phenomena that occurs. Fronts, which are boundaries between different air masses, are characterized by a distinct shift in wind patterns. As a pilot crosses a front, they generally transition from winds associated with one air mass to those associated with another. This often results in a noticeable change in the direction from which the wind is coming, making it a reliable indicator of the front's presence.

The other choices relate to atmospheric changes that may or may not accompany the crossing of a front. For instance, temperature changes often occur as well, but they are not a guaranteed effect since not all fronts involve substantial temperature gradients. Similarly, while cloud cover can change due to different air mass characteristics, conditions like clear skies might persist beyond a front. Lastly, a change in altitude does not have a direct correlation with crossing a front, as pilots may remain at a consistent altitude while traversing different air masses. Thus, the change in wind direction stands out as a definite outcome when crossing a front.