D. So, because if it is added, the sentence given after will become a reason.
' And ' would be used if we are doing two things at the same time.
E.g = I ordered pasta and a glass of red wine.
' But ' would be used if we have an issue but we still made an exception.
E.g = I have a red car but I went by my cycle.
' Or ' is used when given a choice.
E.g = Do you want pepper or chili flakes on your pasta?