Which pattern defines a family of algorithms, encapsulates each one, and makes them interchangeable?

The Strategy Pattern is designed to define a family of algorithms, encapsulating each one so that they can be interchanged easily. This pattern allows a client to choose an algorithm from a family of algorithms at runtime, facilitating the ability to vary the behavior of a class without modifying the class itself.

In practical terms, the Strategy Pattern involves defining an interface that represents the various algorithms. Concrete implementations of this interface then encapsulate the different algorithms, allowing the context to switch between them seamlessly as needed. This promotes flexibility and the principle of composition over inheritance, as you can change the behavior of the application dynamically based on the chosen strategy without altering the core classes.

The other patterns listed serve different purposes. For example, the Facade Pattern simplifies interactions with complex subsystems, the Observer Pattern provides a way to subscribe and notify changes between objects, and the Memento Pattern is used for capturing and restoring an object's state. These purposes do not align with the strategy of defining and interchanging algorithms as seen in the Strategy Pattern.