A candidate key in a relational database is a set of one or more columns that can uniquely identify a row in a table. A primary key is selected from the candidate keys.
- Public key: A public key is used in cryptography for secure communication but is not directly related to DBMS.
- Private key: A private key is also a cryptographic concept and is not associated with DBMS concepts.
- Symmetric key: Symmetric key encryption is a cryptographic concept and is not directly related to DBMS.
- Candidate key: In the context of a DBMS, a candidate key is a set of attributes that uniquely identifies a record in a relation (table). It is a core concept of database design.
Therefore, the correct answer is 4. Candidate key.