If 100 working-class people move upward on the class ladder and, at the same time, 100 middle class people experience downward mobility, sociologists would consider this to be an example of mobility.

The correct answer is exchange mobility. 

Exchange social mobility refers to a sociological phenomenon wherein the number of people moving up and down the social class ladder at a given point in time remain relatively the same. An assumption of this theory is that all social classes comprise of approximately the same number of people.