Final answer:
In a relational database, every record must be uniquely identified by a primary key, which ensures each row is unique and non-null.
Step-by-step explanation:
In a relational database, every record must be uniquely identified by a primary key. A primary key is a special relational database table column (or combination of columns) designed to uniquely identify all table records. A primary key's main features are: it must contain a unique value for each row of data, it cannot contain null values, and it must be designed to guarantee unique values to identify each record uniquely. The other options listed are different types of keys that serve other purposes. A foreign key is used to link two tables together. A composite key is a combination of two or more columns used to create a unique key for a table when a single column is not sufficient to do so. A candidate key is a column, or a set of columns, that can qualify as a primary key in the database.