Often from what we hear from other people,
a) She was the most beautiful girl I had ever seen.
b) She is the most beautiful girl I have ever seen.
Could somebody tell me what is the difference between the above sentences. I often they are interchangeably used. For example, even the girl is still the most beautiful from all I girls I have seen, sometimes people still prefer sentence a instead of sentence b. Sometimes, things get even worse when they mix the tenses up and become
c) She was the most beautiful girl I have ever seen.
d) She is the most beautiful girl I had ever seen.
I often hear sentence c comes out from the native speaker mouth. Is sentence c correct? What about sentence d? Is sentence d correct?