Question

read the excerpt from dr. martin luther kings \letter from birmingham jail.\
just as the apostle paul left his village of tarsus and carried the gospel of jesus christ to the far corners of the greco roman world, so am i compelled to carry the gospel of freedom beyond my own home town.
why does king use this allusion?
to share historical precedent
to emphasize the morality of his cause
to define the purpose of his travel
to compare the goals of famous leaders

Explanation:

An allusion to Apostle Paul (who spread the gospel of Christianity, a moral/religious mission) is used by King to draw a parallel to his own mission of spreading "the gospel of freedom." The moral authority of Paul's religious mission is leveraged to emphasize the morality of King's civil rights cause. Other options: "share historical precedent" is not the focus (the allusion is symbolic, not just historical); "define travel purpose" is incorrect (the purpose is the cause, not travel); "compare goals of leaders" is wrong (it's about the cause's morality, not comparing leaders).

Answer:

B. to emphasize the morality of his cause