Which design principle states that classes should be open for extension but closed for modification?

The principle that states that classes should be open for extension but closed for modification is the Open/Closed Principle. This design principle is foundational in object-oriented programming and emphasizes that software entities such as classes, modules, and functions should be designed in a way that allows their behavior to be extended without altering their existing code.

The rationale behind this principle is to encourage designers to create systems that can evolve over time without requiring significant changes to existing code, which in turn helps to minimize the risk of introducing bugs into a stable codebase. For instance, when new functionality needs to be added, a developer can utilize inheritance or interfaces to extend the existing classes rather than modifying them. This leads to a more maintainable and adaptable software architecture.

In contrast, other design principles have different focuses: the Single Responsibility Principle pertains to ensuring a class only has one reason to change, the Liskov Substitution Principle revolves around substitutability of subclasses, and the Dependency Inversion Principle supports decoupling components within a system. None of these principles specifically encapsulate the idea of being open for extension while being closed for modification, making the Open/Closed Principle the correct answer.