Final answer:
In a database relation, each row's uniqueness is guaranteed by a non-empty Primary key value, which uniquely identifies each row and prevents duplicate records.
Step-by-step explanation:
In a relation, such as a table in a database, each row must be unique. This uniqueness is guaranteed because the relation has a non-empty Primary key value. The primary key consists of one or more columns whose values uniquely identify each row in the table. No two rows can have the same primary key value, which enforces the uniqueness of each row.
Other options like a Candidate key or a Composite key also describe types of keys that can uniquely identify records in a table, but the correct answer to the question is the primary key. A candidate key can be any column, or a set of columns, that can serve as a primary key. When a candidate key is chosen to serve as the unique identifier for rows in a table, it becomes the primary key. A composite key is a type of candidate key that consists of two or more columns.