The correct answer is understatement.
During the first two stanzas, the phrase "one perfect rose" is a positive thing. The final line of the poem, however, turns this phrase on its head. Now, instead of being a positive thing, the use of understatement makes it ironic. Previously, "one perfect rose" was good; by the end of the poem, it has become a disappointment.
The reader initially believes "one perfect rose" is a beautiful expression of love; by the end of the poem, we realize "one perfect rose" is disappointing and, in Parker's estimation, not a great gift.