Which of the following expresses a cause of the increased specialization of labor?
a. economic power of monasteries
b. beginning of a cash economy
c. greater role of banks in lending
d. growth of towns and cities

More of the stated answers could be a cause of the increased specialization of labor. But the most probable answers would be either B - beginning of a cash economy or D - growth of towns and cities.

Once you have big cities, you're also destined to have a lot of speciallization going on. Also, once you have a cash economy, this means there are many goods that need to be compared and taken into account for.