Final answer:
In the context of tornado formation, the best option that fits the empty cell in Timothy's model based on the given choices is 'Tornado develops', as it is the next logical step after a thunderstorm intensifies.
Step-by-step explanation:
Timothy's model represents the steps in the formation of a tornado. When considering the provided context for tornado development, it's clear that tornadoes form from severe thunderstorms, particularly supercells. Supercells are noted for a column of air rotating around a vertical axis, which is a precursor to tornado formation. Therefore, the step that best fits the empty cell in Timothy's model of tornado formation, given the options provided, is B) Tornado develops. This step follows the intensification of a thunderstorm and precedes the actual occurrence of a tornado touching down and causing damage.