Based on the data Martha collected, the barometric pressure is decreasing over the time intervals recorded (from 1012 millibars at 2:00 PM to 996 millibars at 3:30 PM). A falling barometric pressure typically indicates that a low-pressure system is moving into the area, which is often associated with stormy or unsettled weather.
Therefore, the reasonable weather forecast based on Martha's data is:
D. A low pressure system is moving into the area.