here in this case u check every option so i explain one by one
A) AB--->CD , A--->C,C----D
so here candidate key =AB so here A--->C don't satisfied 2NF because here A is proper subset of candidate key so its not in 2nf.
B) similarly CK= AB,AC so B-->C,C--->B,A----->D don't satisfied 2nf
C) similarly Ck= AB, BD. BC
D) CK= AB so here 2nf satified and also check 3nf BC---D is not satified becuase either left side super key or right side prime attribute .......but here both of them don't exist so its not 3NF so option D is correct.
