The answer that is most likely to be correct is the last one: the poem's shape emphasizes the similarities between pears and the people the poem describes.
However, even that option seems to a bit lacking. The shape is complementing the poem's meaning. Why would so many things look like pears? - the speaker asks. Delightful things and people all have some kind of pear-shaped characteristic: the womb, breasts, the cello... The author finds a sort of sensuality in the shape, as she says 'Some say it was a pear Eve ate'.
The shape of the poem comes to show that. The poem, short as it is, is also sensuous, like a woman, like music, like the sweet fruit it alludes to.