The correct answer is: He uses unjust treatment of Sampson by the Philistines to draw attention to the plight of prisoners throughout history.
The author describes both Sampson's and the slave's plights in the story - the slave being discussed in the first two paragraphs, and Sampson being introduced at the last one. Moreover, both these two characters' conditions are presented in a similar manner: They are poor, blind and have been shorn of their strenghts - with Sampson being additionally eager to break his bonds.
The slave in the story is a poetic description of the suffering of every prisoner that has ever existed, and Sampson is included in this characterization as to highlight the prisoner's desire to escape.
