Within the relational model, what best describes a key with respect to a table (relation)? Choose the statement that correctly characterizes a key's role.

Introduction / Context:
A key is central to ensuring entity integrity in a relational table. Understanding what a key actually does—beyond its physical representation—helps avoid design mistakes.

Given Data / Assumptions:

• A key may be single-attribute or multi-attribute (composite).
• The table conforms to relational rules (no duplicate rows under a candidate/primary key).
• We distinguish a key’s purpose from its implementation details.

Concept / Approach:
The purpose of a key is to uniquely identify each row (tuple). Whether this key is one column or multiple columns is incidental; the defining property is uniqueness and minimality (for candidate keys).

Step-by-Step Solution:

Verification / Alternative check:
Relational design guidelines define a key as a unique identifier for tuples.

Why Other Options Are Wrong:
Must be two or more columns / can only be one column: Both are false—keys can be either.
Identifies a column: Keys are constraints over columns; they don’t identify columns.
Used only for sorting: Sorting is unrelated; keys provide uniqueness and referential anchors.

Common Pitfalls:
Believing all keys must be composite or single—design drives the choice.

Final Answer:
identifies a row.