The correct answer would be structure. Many people think it to be rhyme, due to the word pattern, however, this is incorrect. The structure of a poem is how the poem is presented to the reader. Small, detailed things, such as formatting (form), as well as the flow (pattern) of the words chosen and/or ideas conveyed to the reader.
Anyway, I know I may be really late on answering, but hopefully people who search this question in the future will find this helpful. :)