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.
What are the choices given here? Anyway, if the question is whether the sentence "The couple looked closely at the exterior of the house, they noticed; the paint was in disrepair." uses correct punctuation, I shall answer that it does not.
It could possibly be: The couple looked closely at the exterior of the house; They noticed the paint was in disrepair.
