In "To a Poor Old Woman," the plums represent the simple joys of life, which are available to anyone.
In this poem, the narrator observes a “poor old woman” eating plums from a paper bag, which makes him think about its taste. He knows that plumps are not really expensive neither extraordinary, and that's why he supposes that the woman is poor, since she seems not to have eaten plums in a long time.
This represents a joy of life for her, while for other people it's something normal.
The right answer is the D: They represent the simple joys of life, which are available to anyone. The old woman is eating the ripe plums from a bag and on the street, and she is eating them (not just one, but a few) with gusto, enthusiastically ("... by / the way she gives herself / to the one half / sucked out in her hand"), which could be suggesting that she is poor and hungry, yet she finds comfort and solace in them, and that's good enough for her. No matter what others, more privileged than her, may have at their disposal: she, too, has access to this simple joy of life, a bag of juicy and tender plums, which are providing a pleasurable moment that no one can deny to her.
