Which statement regarding indexes is correct?

Unique indexes serve a critical function in a database by ensuring that all values in a column (or a combination of columns) are unique. This means that when you create a unique index on a specific attribute, the database will reject attempts to insert duplicate values in that column, effectively preventing data duplication. This is particularly useful in maintaining data integrity and consistency within the database, as it enforces rules about the uniqueness of certain records.

The other statements present inaccuracies about how indexes function or their characteristics. Custom indexes can typically be added to entities in many database systems. While indexes do improve the speed of data retrieval, they can also consume additional storage space and may slow down data modification operations such as inserting or updating records. Additionally, while it is common practice to manage indexed attributes, there are scenarios where indexes on referenced attributes can indeed be deleted, depending on the database management system's policies and constraints.