The correct answer is (a) storm.
A sudden drop in atmospheric pressure indicates that a storm is approaching. This is because the low pressure system associated with a storm draws in air from the surrounding area, causing the pressure to drop. The lower pressure can also cause the wind to pick up and the temperature to drop.
(b) Rain is not always associated with a sudden drop in atmospheric pressure. Rain can occur for a variety of reasons, including high humidity, warm temperatures, and the presence of clouds.
(c) Clean weather is not always associated with a sudden drop in atmospheric pressure. Clean weather can occur for a variety of reasons, including low humidity, cool temperatures, and the absence of clouds.
(d) A cold wave is not always associated with a sudden drop in atmospheric pressure. A cold wave can occur for a variety of reasons, including the movement of cold air from the north, the presence of high pressure systems, and the absence of clouds.