a. Valerie is the least experiencedest artist ever to win the contest.
b. Valerie is the experiencedest artist ever to win the contest.
c. Valerie is the least experienced artist ever to win the contest.
d. Valerie is the less experienced artist ever to win the contest
In A, 'experiencedest' is not a word. In B, once again it uses 'experiencedest.' In D, it says "Valerie is the less experienced artist" which not correct grammar.