Which of the following is NOT one of the four pillars of object-oriented programming?

The correct answer is C, as "Algorithm" is not considered one of the four pillars of object-oriented programming (OOP). The four main pillars of OOP are encapsulation, polymorphism, inheritance, and abstraction.

Encapsulation refers to the bundling of data and methods that operate on that data within a single unit or class, restricting access to some of the object's components. This is important for protecting the integrity of the object's data and simplifying the interface for interaction.

Polymorphism allows for methods to be used in different contexts, meaning that the same function name can apply to different types (or classes) of objects, providing flexibility and the ability to override methods in derived classes.

Inheritance is a mechanism for creating new classes from existing ones, promoting code reusability and establishing a hierarchical relationship between classes.

While algorithms are fundamental in programming, they pertain to the methods and procedures for solving problems rather than to the design paradigm of OOP. Therefore, "Algorithm" does not fit within the framework of the four pillars of object-oriented programming.