Which of the following can be considered a foundational concept of object-oriented design?

Polymorphism is considered a foundational concept of object-oriented design because it allows objects of different classes to be treated as objects of a common superclass. This capability enables a single interface to control access to different functions in the underlying objects, providing flexibility and making code easier to extend and maintain. In object-oriented programming, polymorphism supports the design principle of "programming to an interface, not an implementation," which enhances code reusability and adaptability.

In contrast, coding standards, error handling, and testing frameworks, while important in software development, are not core principles of object-oriented design itself. Coding standards refer to guidelines for writing code consistently, which helps maintain readability and quality. Error handling pertains to the robust management of errors during program execution, ensuring the application can respond gracefully to unexpected situations. Testing frameworks are tools that aid in the verification and validation of code, supporting the software development lifecycle but not specifically defining the object-oriented design paradigm.