Which internal table type allows unique and non-unique keys?

The correct answer is that a sorted internal table allows for unique and non-unique keys. Sorted internal tables maintain their entries in ascending order based on a specified sort criteria and support both unique and non-unique keys.

When using a sorted internal table, you can define a unique key, which means that no two entries can have the same key value. However, you can also create it with non-unique keys, allowing multiple entries to have the same key value but still retain order. This flexibility is particularly useful when needing to maintain sorted data while also allowing for duplicate entries when necessary.

In contrast, the other options specialize in different ways of handling data. Hashed tables are designed primarily for fast access and use a hashing algorithm, which does not inherently support maintaining any particular order of entries or allow duplicate keys. Standard tables do allow duplicate entries and do not maintain order but do not provide key constraints. The flexible option does not represent a standard type in ABAP terminology specifically, therefore it is not applicable in this context.