Since Maria's husband died during 2014, the IRS still considers her married for the entire 2014 tax year.
Assuming she did not remarry, she should generally file Married Filing Jointly with her deceased husband. This usually provides a better tax rate and a larger standard deduction than filing separately.
She does have a choice. For 2014, she can generally file either:
Married Filing Jointly, or
Married Filing Separately
She cannot use Qualifying Widow filing status for 2014 because that status applies to the two years after the year her spouse died, assuming she meets the requirements.
For the 2014 return, she can use Form 1040 or Form 1040A, depending on her income and other tax circumstances.
So the best answer is generally Married Filing Jointly for 2014.