Final answer:
Sean and Cynthia would likely file as 'Single' or 'Head of Household' after their divorce on September 21, 2011, as they can no longer file jointly according to IRS rules.
Step-by-step explanation:
The filing status of Sean and Cynthia, who were divorced on September 21, 2011, would depend on various factors, including whether they have any dependents and if they entered into any specific tax filing agreements post-divorce. However, as per Internal Revenue Service (IRS) rules, since they were divorced before the end of the year, they cannot file jointly for that tax year. Each individual would typically file as Single or Head of Household if they qualify. To file as Head of Household, one must have paid more than half of the household expenses, be considered unmarried for the tax year, and have a qualifying dependent living at home for more than half the year.