The correct answer is D) a cold air mass meeting a warm air mass, forming a stationary front over the region.
The weather map shown in the image forecasts the weather over a region. What causes the weather conditions shown in the map is "a cold air mass meeting a warm air mass, forming a stationary front over the region."
In Wheater terminology, a stationary front forms when to air masses collapse and none of those air masses has more power to advance over the other. It could be that warm or cold front slows down below the six miles per hour to create this stationary front. This front could cause many different weather conditions, depending on the humidity or the dryness.