Final answer:
The majority of tornadoes in the United States occur in the spring of the year. Therefore, the correct option is a) Spring.
Step-by-step explanation:
The majority of the approximately 1300 tornadoes that are reported each year in the United States occur in the spring of the year. Tornadoes are violent and destructive weather events that form from severe thunderstorms called supercells. The rotation of air in these supercells creates a column of rotating air that can shift and eventually produce a tornado.