Question

and set the war between us once again.
we keep the wall between us as we go.
to each the boulders that have fallen to each.
and some are loaves and some so nearly balls
we have to use a spell to make them balance:
\stay where you are until our backs are turned!\
we wear our fingers rough with handling them.
oh, just another kind of out - door game,
one on a side. it comes to little more:
there where it is we do not need the wall:
he is all pine and i am apple orchard.
what does the phrase \one on a side\ mean?
the neighbor repairs his side of the wall while the speaker watches.
the speaker and the neighbor repair the wall from the same side.
the speaker repairs his side of the wall while the neighbor watches.
the speaker and the neighbor repair the wall from opposite sides.

Explanation:

The phrase "one on a side" is used in the context of repairing a wall. Since the speaker and the neighbor are working on the wall, and the previous lines mention "We keep the wall between us as we go" and "To each the boulders that have fallen to each", it implies that they are on opposite sides of the wall. Each is responsible for their own side (the side that the boulders have fallen to them). The other options are incorrect as there is no indication of one watching while the other repairs (options 1 and 3) and "same side" (option 2) contradicts the idea of the wall being between them.

Answer:

The speaker and the neighbor repair the wall from opposite sides.