Candidate keys are ABD,BCD.
A candidate key is minimal super key which uniquely identifies a relation.
There can be many superkeys and out them candidate key is selected.
ABD+ = { A,B,C,D,E,F}
BCD+ = { A,B,C,D,E,F}
Now prime attributes are the one that are present in candidate key.
Non prime attributes are E,F
Here A,B,C,D are Prime attributes.
FD1 : AB->C ( part of key -> prime attribute) --->3NF
FD2 : DC->AE(part of key ->nonkey) ------>1NF
FD3 : E->F (nonkey->nonkey)-------->2NF.
So highest normal form of complete Relation is 1NF.