Final answer:
A couplet is comprised of two adjoining lines that share an end rhyme, often used to underline the theme of a poem, particularly in sonnets. Heroic couplets and split couplets are variations that have different metrical patterns or serve specific thematic purposes in poetry.
Step-by-step explanation:
A couplet is a group of two adjoining lines of poetry which share an end rhyme. It is often used to reiterate the theme of a sonnet or to provide a concluding thought. The correct answer to what a couplet is would therefore be: A group of two lines that rhyme and reiterate the theme of the sonnet. An example of a couplet can be found in the final two lines of Shakespeare's Sonnet 18:
So long as men can breathe or eyes can see,
So long lives this, and this gives life to thee.
Other forms of stanzas include the tercet (three lines), quatrain (four lines), cinquain (five lines), and sestet (six lines). Heroic couplets, popular in the 17th and 18th centuries, are iambic pentameter lines that rhyme in pairs, like those by Alexander Pope. A ghazal, however, is composed of a series of couplets that are thematically autonomous, and a split couplet differs in its metrical structure between the two lines.