Hello, the correct answer would be either A or D. B cannot be correct because it still remains a fragment. The same case is with C. A might be correct in the following sense: Since, they had traveled often. "Since" meaning - after that. And D might also be correct because if you delete "since", you are left with - "They had traveled often", which is a completely correct and completed sentence.
The answer is C
"me" is a first person pronoun :)
