The phrase "one on a side" in the poem refers to the idea that there is one person on each side of the wall repairing it. The speaker and the neighbor are working on their respective sides of the wall, emphasizing their individuality and separation. This symbolizes the division between them and their different perspectives on the need for the wall.
By choosing the option "The speaker repairs his side of the wall while the neighbor watches," you correctly identify that the phrase "one on a side" implies that each person is responsible for repairing their own side of the wall. This interpretation highlights the thematic significance of boundaries, autonomy, and differing beliefs portrayed in the poem.