Which programming language is not based on object-oriented design principles?

C is the programming language that is not based on object-oriented design principles. Unlike Java, C#, and Ruby, which are designed with object-oriented paradigms in mind, C is primarily a procedural programming language.

In C, the focus is on writing functions and procedures to operate on data, as opposed to encapsulating data and behavior within objects. While you can implement some object-oriented concepts in C using structures and function pointers, it lacks native support for essential object-oriented features such as inheritance, polymorphism, and encapsulation.

On the other hand, languages like Java, C#, and Ruby are built around the object-oriented model. They provide mechanisms for creating classes and objects, managing access to data through encapsulation, enabling inheritance to share properties and methods between classes, and allowing polymorphism for method overriding and interfaces.

This fundamental structural difference makes C distinct from the others listed, clearly categorizing it as a non-object-oriented language, while the others are deeply rooted in object-oriented programming.