Which pattern provides a flexible alternative to subclassing for extending functionality?

The Decorator Pattern offers a flexible alternative to subclassing by allowing behavior to be added to individual objects at runtime without affecting the behavior of other objects from the same class. This pattern achieves this by wrapping an object with a new class that implements the same interface and adds additional responsibilities or functionalities.

Instead of creating a multitude of subclasses to cover various combinations of functionality, the Decorator Pattern promotes composition over inheritance. This allows developers to change an object's behavior dynamically, which is particularly useful when you want to extend functionalities in a modular way. As a result, you can combine decorators in various ways to achieve a wide range of behaviors while maintaining simpler and more manageable code.

Other patterns mentioned, such as the Strategy Pattern, primarily focus on defining a family of algorithms, encapsulating them, and making them interchangeable. The Singleton Pattern restricts the instantiation of a class to a single instance, which does not relate to extending functionality. The Prototype Pattern is used for creating new objects by copying an existing object rather than extending behavior through composition. Thus, while they serve their purposes effectively, they do not provide the same flexible means for extending functionality as the Decorator Pattern does.