Members of the middle class belong to the A. bourgeoisle
B. aristocracy
C. peasantry
D. proletariat
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A.bourgeoisle

Explanation:

The bourgeoisle is the best option since bourgeoisle means a middle class and the social defined class for the bourgeoisle was known as the middle class. It couldn't be B because aristocracy was defined as the richest class, the peasantry was the opposite, it was defined as the poor class. The proletariat class was the working class who was also the lowest class. Therefore, its A.