Which of these people should use the "married filing jointly" tax filing stat

You may qualify for more than one tax filing status. For example, a single (unmarried) person isn't necessarily restricted to the single filing status. He or she may be able to file as head of household or qualifying widow (er), assuming those qualifications are met. But if he or she wasn't legally married as of 12/31/2015, they couldn't file their 2015 return as a married person.