Which Object-Oriented Programming principle refers to bundling data (attributes) and the methods that operate on that data into a single unit, while restricting direct external access to some of that data?

Encapsulation bundles an object's data and the methods that act on it within a single class, and uses access specifiers (like private and protected) to restrict direct external access to the internal state — exposing controlled access only through public methods. Inheritance is about a class acquiring properties from a parent class. Polymorphism allows a method or operator to behave differently depending on context. Abstraction focuses on hiding implementation complexity and exposing only essential features, and Composition describes building complex objects out of other objects, which is a different relationship altogether.