The correct answer is option 1 Concept: Referential integrity constraints In a relational database, a referential integrity constraint is specified between two tables with the help of a foreign key. In the Referential integrity constraints, if a foreign key in Table 1 refers to the Primary Key of Table 2, then every value of the Foreign Key in Table 1 must be null or be available in Table 2. B is a foreign key of R1 that refers to C(primary key) in R2, B must be null or to be available in R2 r2(C) is a superset of r1(B) So {subset - Superset} =Φ So the correct answer is ΠB(r1) - ΠC(r2) = Φ Additional Information functional dependency: Dependency of attributes in relations Foreign key A foreign key means that values in one table must also appear in another table. T he referenced table is called the parent table while the table with the foreign key is called the child table. The foreign key in the child table will generally reference a primary key in the parent table. Primary key A primary key is a specific choice of a minimal set of attributes (columns) that uniquely specify a tuple (row) in a relation (table). Primary key cannot accept null values and there can be only one primary key constraint for one table.