Final answer:
The midwestern United States, also known as Tornado Alley, has the greatest frequency of tornadoes due to its unique atmospheric conditions.
Step-by-step explanation:
The region with the greatest frequency of tornadoes is C. the midwestern United States. This area is often referred to as Tornado Alley and is known to storm chasers as the most active part of the world for tornadoes. The unique geographical and atmospheric conditions between the Rocky Mountains and the Appalachian Mountains create an environment conducive to the development and formation of tornadoes. Tornadoes descend from clouds in funnel-like shapes and spin violently, posing significant risks to property and life in these regions.