What best describes a foreign key in relational databases?

• A. A column that uniquely identifies a row within the same table.
• B. An index that speeds up queries on a column.
• C. A constraint that enforces domain integrity on a column.
• D. The primary key of one table that appears as an attribute in another table and acts to provide a logical relationship between the two tables.

Answer: (D)

A foreign key is a field (or set of fields) in one table that refers to the primary key of another table, creating a link between the two tables and enabling referential integrity. This is exactly what the described option conveys: the primary key of one table appearing as an attribute in another table to establish a relationship between the tables. This linkage allows related data to be connected and ensures that values in the foreign key correspond to existing rows in the referenced table.

The other concepts are different: a field that uniquely identifies a row within the same table describes a primary key rather than a relationship to another table; an index speeds up queries but does not establish cross-table links; a constraint that enforces domain integrity governs valid values within a single column rather than linking two tables.