An economic reason for the institution of
serfdom was to maintain
(1) an educated citizenry
(2) a stable workforce
(3) overseas trade
(4) religious uniformity

An economic reason for the institution of serfdom was to maintain "(2) a stable workforce" although it should be noted that there were other reasons as well. 

The correct answer is (2) a stable workforce

Without serfdom, people wouldn't be forced to work and manors and similar things might have fallen apart. This way, the class of serfs existed and children would grow being serfs forever and would just replace their parents as workers and all was well for the nobility. Eventually the serfs were liberated and didn't have to do what they were told.

The answer is d.  the north had organized schools, whereas the south had few organized schools.