Danny Casey is one of the best players. (positive degree)

(A) Very few players are as good as Danny Casey. CORRECT. Superlative "one of the best" → Positive degree: "Very few ... are as good as ..."

(B) Danny Casey is better than any other players. WRONG. "Better than" is Comparative degree, not Positive.

(C) No other player is so good as Danny Casey. WRONG. "No other" is a superlative-type positive; the standard GSEB answer is "Very few ... as good as..."

(D) Danny Casey is better than most other players. WRONG. "Better than most" is Comparative degree, not Positive.