Final answer:
The given relation violates BCNF. To modify it, the relation needs to be decomposed into two new relations: R1(A, B, C) and R2(A, D).
Step-by-step explanation:
In this case, the given relation violates the Boyce-Codd Normal Form (BCNF).
The BCNF violation occurs because there is a functional dependency (A, B) → C, where the determinant (A, B) is not a superkey for the relation.
To modify the relation to be in BCNF, we need to decompose it into two new relations: R1(A, B, C) and R2(A, D), where the primary keys of the new relations include the attributes that determine their non-key attributes.