Which language is not built on object-oriented design principles?

C is the correct answer because C is a procedural programming language, and it does not incorporate the principles of object-oriented design. In contrast, Java, C++, and Python are all designed to support object-oriented programming (OOP) concepts such as classes, objects, inheritance, encapsulation, and polymorphism.

While C++ is an extension of C and indeed supports OOP alongside its procedural features, and Java and Python are fully object-oriented from the outset, C focuses on structured programming and does not provide constructs for creating objects or classes, which are fundamental to OOP. The absence of these features in C makes it distinct from the other languages listed that embrace and utilize object-oriented principles effectively.