Final answer:
Pilots should avoid penetrating severe frontal weather such as the cold front, occluded front, and pre-frontal squall line due to the risk of strong winds, heavy rain, and thunderstorms.
Step-by-step explanation:
The worst frontal weather that pilots should probably not attempt to penetrate includes the cold front, the occluded front, and the pre-frontal squall line. These weather fronts are associated with severe weather conditions like strong winds, heavy rain, thunderstorms, and sometimes hail. A cold front can lead to a rapid drop in temperature, and an occluded front occurs when a cold front overtakes a warm front, leading to complex weather patterns. A squall line is a line of severe thunderstorms that can form ahead of a cold front, bringing intense rain and wind.