In a relational database, a ___________ allows efficient retrieval of records based on specific criteria.

An index in a relational database is a data structure that allows for efficient retrieval of records based on specific criteria. It acts as a pointer to the data in the table and can significantly speed up queries by providing quick access to rows that meet certain conditions. A primary key is a specific type of index that uniquely identifies each record in a table. A foreign key is used to establish relationships between tables. A unique key ensures that no two records have the same values in specified columns. While all these options are related to data retrieval and optimization, an index is specifically designed for efficient search operations, making it the most suitable choice.