Correct Answer is option D IMO.
Although Jane's father passed away more than 15 years ago, she still pays an annual visit to his [u]grave in Minnesota once a year because she feels that it is appropriate and respectful.
A. grave in Minnesota once a year because she feels that it is appropriate and respectful
INCORRECT
'It' is an orphan pronoun in the original construction.
B. grave in Minnesota because she feels that it is appropriate and respectful
INCORRECT
Phrase 'once a year' is missing in this construction.
C. grave, which is located in Minnesota, since she feels that doing it is appropriate and respectful
INCORRECT
Phrase 'once a year' is missing in this construction.
D. grave in Minnesota once a year because she feels that it is appropriate and respectful to do so
CORRECT
E. grave in Minnesota since she feels that doing so is respectful and appropriate
INCORRECT
Phrase 'once a year' is missing in this construction.