The correct answer is B.
In these lines, Cullen describes men who plant, but do not reap. Instead, the fruits of their labors ("the golden increment of bursting fruit") is taken by others. Thus, the men labor with no reward. However, Cullen says it will not always be so.
Furthermore, he says that they will not always stand by silently while lesser men hold their brothers cheap. These "lesser men" are those who reap the fruits of other men's labors and in so doing degrade them. In degrading others, they degrade themselves and thus become "lesser."
Therefore, these lines describe the paradox of social injustice: that degrading others ultimately causes a person to become inferior to those he degrades. The correct answer is B.
