Final answer:
Stormy weather is most likely to occur when a cold front moves into an area. This weather event is associated with a drop in temperature, wind changes, and the potential for precipitation as the cold front lifts the warmer air, leading to cloud formation and storms.
Step-by-step explanation:
When a cold front moves into an area, it typically brings a drop in temperature and a change in wind direction. As this colder air mass moves in, it wedges underneath the warmer air that it's replacing, which leads to the warmer air being lifted upwards. This process can cause stormy weather due to the development of clouds and precipitation as the lifted air cools and condenses. After the initial stormy conditions, the skies may clear, but stormy weather is most associated with the arrival of a cold front.
Considering the other options provided, stormy weather (A) is what one would most likely expect when a cold front arrives. Warm weather (B) would be uncharacteristic because the defining feature of a cold front is the introduction of cooler air. Sunny weather (C) might follow after the front has passed and the atmosphere stabilizes, but it is not the most immediate effect. Cloudy weather (D) is often a part of the stormy conditions a cold front can bring but doesn't fully describe the potential for precipitation and thunderstorms that can accompany the front.