Which key, selected from among the candidate keys of a table, is used to uniquely identify each record and is automatically enforced by the DBMS to disallow NULL values?

The Primary Key is the candidate key chosen to uniquely identify each row in a table; the DBMS automatically enforces both uniqueness and a NOT NULL constraint on it. A Foreign Key references a primary key in another table and can contain NULLs. An Alternate Key is any candidate key not chosen as the primary key. A Composite Key is a key made of multiple columns which may or may not be the primary key, and a Super Key is any set of attributes that can uniquely identify a row but may include unnecessary extra attributes.