regard/ your application/ regret/ inform/ vacancy/ already/ fill.
A.As regards your application, I regret to inform you that the vacancy has already been filled.
B.As regards to your application, I regret to inform you that the vacancy has already been filled.
C.As regards your application, I regret to inform you that the vacancy has already filled.
D.As regards your application, I regret informing you that the vacancy has already been filled.