To solve this question, the user needs to know the definitions of each weather system and how they are classified.

A. Hurricane: A hurricane is a tropical cyclone that occurs in the Atlantic Ocean and northeastern Pacific Ocean.

B. Typhoon: A typhoon is a tropical cyclone that occurs in the northwestern Pacific Ocean.

C. Tropical cyclone: A tropical cyclone is a rotating low-pressure weather system that has organized thunderstorms but no fronts (a boundary separating two air masses of different densities).

D. Tornado: A tornado is a rapidly rotating column of air that is in contact with both the surface of the Earth and a cumulonimbus cloud or, in rare cases, the base of a cumulus cloud.

Therefore, option D which is the Tornado is the smallest amongst all the weather systems listed. Tornadoes have a much smaller size than other weather systems such as hurricanes, typhoons, and tropical cyclones, which can span hundreds of miles and have a significant impact on large regions.

The Answer is: D. Tornado