What is a key difference between abstract classes and interfaces?

The key difference highlighted in the correct choice is that abstract classes can provide method implementations while interfaces cannot. Abstract classes are designed to allow for some common functionality to be defined, enabling derived classes to either use or override these implementations. This makes them versatile in scenarios where a base class behavior is necessary, while still allowing inheritance and polymorphism.

In contrast, interfaces are purely a contract that defines what methods a class must implement, without providing any default behavior. This allows for a diverse set of classes to implement the same interface regardless of their position in the class hierarchy, supporting a different approach to designing systems with multiple behaviors.

Understanding this distinction is important because it impacts design choices in object-oriented programming. Abstract classes allow for shared code and the enforcement of certain behaviors, while interfaces promote flexibility and multiple inheritance of behavior from unrelated classes.

The other choices lack a key understanding of object-oriented principles. For instance, both abstract classes and interfaces can contain abstract methods, but only abstract classes can have concrete methods. Also, in both structures, variables can exist with different accessibility modifiers, and the rules around extensions and inheritance for classes and interfaces differ fundamentally in terms of implementation.