BCNF is not used for cases where a relation has
A) possible for BCNF. As in BCNF in X-> a transition we take all X as super key and it should be in 3 NF
Say a relation AB -> CD
AC -> BD
AD -> BC
All left side are candidate keys and it is in 3NF , So also in BCNF
D) Mutually exclusive foreign keys not a problem for that relational table and also constructing a BCNF
Composite Key is a key which is combination of more than one field or column of a given table. It can be a Candidate key, Primary key. So, a table with combination of its primary key and candidate key can be called as composite key.
AB->CD and AC->BD. These keys are composite as well as candidate and the relation is in BCNF.
I think the answer must be d by option elimination but i don't really get the meaning of it.