The rhyme scheme of the second quatrain of Shakespeare's "Sonnet 100" is abab. The lines are:
- Return, forgetful Muse, and straight redeem (a)
- In gentle numbers time so idly spent; (b)
- Sing to the ear that doth thy lays esteem (a)
- And gives thy pen both skill and argument. (b)
So the correct answer is abab.