Which Python data structure is mutable?

Lists are a fundamental data structure in Python that allows for the storage of ordered collections of items. The key characteristic of lists that makes them mutable is the ability to change their content without creating a new list. This means you can add, remove, or modify elements in a list after it has been created.

For instance, you can append new items using the append() method, remove items with the remove() method, or even change an existing item by accessing it via its index. This mutability is in contrast to the other options, where tuples, strings, and sets have either fixed length, content, or specific rules regarding modifications, all of which prevent direct alterations to the data structure itself.

In summary, lists stand out as mutable data structures in Python, enabling dynamic handling of collections, which is essential for many programming scenarios.