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B is corrent. The requirement is to determine in which situation may taxpayers file as married filing jointly. Taxpayers can choose married filing jointly as their filing status if they are married on the last day of the year and both spouses agree to file a joint return. It doesn’t matter if they lived apart during the year. Taxpayers are considered unmarried for the whole year if, on the last day of the year, they are divorced or are legally separated under a divorce or legal separate maintenance decree. It does not matter if they lived together for the entire year.
A is incorrect because taxpayers who were married but lived apart may file a joint return. 
A is incorrect because taxpayers who were married but lived apart may file a joint return. 
D is incorrect because taxpayers who were married but lived apart may file a joint return.