The correct answer is Primary key constraint. A primary key constraint uniquely identifies each row in a database table and cannot be null. It is used to enforce the uniqueness of the row data and serve as an identifier for referencing the rows. A Unique constraint ensures that all values in a column are different but does not prevent null entries. Foreign key constraint is utilized to maintain the referential integrity between two tables, not for uniqueness within one table. The Check constraint is used to specify a predicate that all tuples must satisfy, but it does not enforce uniqueness nor prevent nulls by itself.