Final answer:
The underlined section at the end of Shakespeare's Sonnet 100 is known as a couplet, which is two adjoining lines of poetry that rhyme and provide closure to the sonnet.
Step-by-step explanation:
When Shakespeare's Sonnet 100 is analyzed, the section at the end of the poem with only two lines is identified as a couplet. A couplet consists of two adjoining lines of poetry that typically rhyme with each other, which is a common feature in Shakespeare's sonnets. Unlike the quatrain, which is a stanza of four lines, a couplet is more concise. Considering the structure of a Shakespearian sonnet, which includes three quatrains followed by a final couplet, the underlined section at the end of Sonnet 100 is definitely a couplet. This is supported by the distinct feature of Shakespeare's sonnets ending with a rhyming couplet, which offers a sense of closure or summary to the poem.