In the context of your question about the purpose of the author in the poem "Paul Revere's Ride," the correct answer would likely be to illustrate the consequences of losing the war to the British.
The reason for this is that by comparing "mast and spar" to a "prison bar," Longfellow effectively conveys a sense of confinement or loss, implying that defeat in the war would lead to a loss of freedom and the hope of independence.
Therefore, you can place the phrase "prison bar" in the first blank and "illustrate the consequences of losing the war to the British" in the second blank.