Which of the following describes a foreign key in the context of entity relationships?

A foreign key is a crucial concept in the context of entity relationships within database design. It serves as a link between two entities by referencing a primary key from another entity, thereby establishing a relationship between them.

The correct choice highlights that a foreign key creates a link to a single row in another entity. This means that each instance of the entity containing the foreign key is associated with one specific instance of the related entity. For example, in a database representing customers and orders, each order might contain a foreign key that links back to a specific customer in the customer table. This structure ensures that the relationship maintains referential integrity—ensuring that each order can be traced back to exactly one customer.

The understanding of foreign keys is critical for establishing relational integrity and enforcing the principle that certain relationships should remain consistent across the database.