Question

Where do tornadoes most commonly form?

Answer 1

Tornado Alley

Explanation:

While tornadoes can form anywhere in the world, they most commonly form in the Great Plains states of the US, more commonly referred to as "Tornado Alley."
(https://www.ncdc.noaa.gov/climate-information/extreme-events/us-tornado-climatology/tornado-alley)

These states are ideal for tornadoes because they tend to be very flat and dry (not many lakes/rivers), which allows cool, dry winds from the north and warm, moist winds from the south to meet and form tornadoes.
(http://www.spc.noaa.gov/faq/tornado/)