If researchers find that the great recession of 2008 led to large numbers of middle-class people experiencing downward mobility to the lower-middle class, it would be an example of what?

The researchers find that the Great Recession of 2008 led to large numbers of middle-class people experiencing downward mobility to the lower-middle class is an example of structural mobility.

Explanation:  

Structural mobility can be defined as a type of a vertical mobility. It is the kind of mobility that manages to take place when there occurs a change in the stratification hierarchy itself. It refers to specifically the movement of a group in a vertical manner, downward in this case.

This happens to a specific group or class, here being the middle class which caused the whole middle class to move downward in a vertical manner making it a structural mobility, making it the right answer.

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