Question: In which poetic form is Mad Girl's Love Song written and appears in the novel The Bell Jar by Sylvia Plath?

Context:
Letters Home: Correspondence 1950–1963
Letters Home is a collection of letters written by Sylvia Plath to her family between her years at college, in 1950, and her death at age 30.
 Sylvia's mother, Aurelia Schober Plath, edited the letters and the collection was published by Harper & Row (US) and Faber & Faber (UK) in 1975.
Mad Girl's Love Song
"Mad Girl's Love Song" is a poem written by Sylvia Plath in 1951, while she was a student at Smith College.
 It is written in the villanelle poetic form and is generally included in the biographical note appended to Plath's novel, "The Bell Jar".
Winter Trees
Winter Trees is a 1971 posthumous collection of poetry by Sylvia Plath, published by her husband Ted Hughes.
 Along with "Crossing the Water" it provides the remainder of the poems that Plath had written during her state of elevated creativity prior to her suicide.
Hypoxia (Kathryn Williams album)
Hypoxia is Kathryn Williams 12th album and was released by One Little Indian on 15 June 2015.
 The songs were initially conceived as a result of a 2013 writing commission from New Writing North in conjunction with the Durham Book Festival's 50th anniversary celebration of the publication of Sylvia Plath's 'The Bell Jar' novel.
Muwashshah
Muwashshah (Arabic: موشح‎ ‎ "muwaššaḥ "  literally means "girdled" in Classical Arabic; plural "muwāshshaḥāt " موشحات or "tawāshīḥ " تواشيح ) is the name for both an Arabic poetic form and a secular musical genre.
 The poetic form consists of a multi-lined strophic verse poem written in classical Arabic, usually consisting of five stanzas, alternating with a refrain with a running rhyme.
 It was customary to open with one or two lines which matched the second part of the poem in rhyme and meter; in North Africa poets ignore the strict rules of Arabic meter while the poets in the East follow them.
 The musical genre of the same name uses "muwaššaḥ" texts as lyrics, still in classical Arabic.
 This tradition can take two forms: the "waṣla" of Aleppo and the Andalusi "nubah" of the western part of the Arab world.
Crossing the Water
Crossing the Water is a 1971 posthumous collection of poetry by Sylvia Plath that was prepared for publication by Ted Hughes.
 These are transitional poems that were written along with the poems that appear in her poetic opus, "Ariel".
 The collection was published in the UK by Faber & Faber (1975) and in the USA by Harper & Row (1976).
The Bell Jar
The Bell Jar is the only novel written by the American writer and poet Sylvia Plath.
 Originally published under the pseudonym "Victoria Lucas" in 1963, the novel is semi-autobiographical, with the names of places and people changed.
 The book is often regarded as a "roman à clef" since the protagonist's descent into mental illness parallels Plath's own experiences with what may have been clinical depression.
 Plath died by suicide a month after its first UK publication.
 The novel was published under Plath's name for the first time in 1967 and was not published in the United States until 1971, in accordance with the wishes of both Plath's husband, Ted Hughes, and her mother.
 The novel has been translated into nearly a dozen languages.
 The novel, though dark, is often read in high school English classes.
Landay (poetry)
The landay is a traditional Afghan poetic form consisting of a single couplet.
 There are nine syllables in the first line, and thirteen syllables in the second.
 These short poems typically address themes of love, grief, homeland, war, and separation.
 The poetic form, traditionally sung aloud, was likely brought into Afghanistan by Aryan nomads thousands of years ago.
 "Landay," in Pashto, means "short, poisonous snake", likely an allusion to its minimal length and use of sarcasm.
Bell jar
A bell jar is a glass jar, similar in shape to a bell, and can be manufactured from a variety of materials (ranging from glass to different types of metals).
 Bell jars are often used in laboratories to form and contain a vacuum; they may also serve as display cases or transparent dust covers.
The Bell Jar (film)
The Bell Jar is a 1979 film based on Sylvia Plath's 1963 book "The Bell Jar".
 It was directed by Larry Peerce, and stars Marilyn Hassett and Julie Harris.
 The story follows a young woman's summer in New York City working for a women's magazine, her return home to New England, and her subsequent psychological breakdown within the context of the difficulties of the 1950s—ranging from the Rosenbergs' execution, to the disturbing aspects of pop culture, to the distraction of predatory college boys.

Answer: the villanelle poetic form