What is a key characteristic of encapsulation in object-oriented design?

A key characteristic of encapsulation in object-oriented design is protecting an object's state by restricting access. Encapsulation is a fundamental principle that ensures an object's internal state and behavior are hidden from the outside world. This is typically achieved by defining access modifiers such as private, protected, or public. By restricting access to the internal data, encapsulation helps in maintaining control over how that data is accessed or modified.

This protection not only safeguards the integrity of the object's data but also enables developers to change the implementation details without impacting outside code. External entities interact with the object exclusively through a defined interface, commonly consisting of public methods that provide controlled access to the object's properties and behaviors, thereby promoting data hiding and reducing the likelihood of unintended interference.

In contrast, the other options do not align with the principle of encapsulation. While code duplication and combining multiple classes into one suggest ineffective or inappropriate design principles, exposing all internal methods contradicts the essence of encapsulation, which is about limiting access to internal structure.