Step 1: Recall the four pillars every OOP textbook lists.
Object Oriented Programming is traditionally built on four foundational principles, encapsulation, abstraction, inheritance, and polymorphism, these four ideas together define what makes a language or design object oriented.
Step 2: Match each option to a pillar.
Encapsulation bundles data with the methods that operate on it and hides internal details, abstraction hides complex implementation and exposes only essential features, and polymorphism lets the same interface behave differently depending on the actual object type, all three are directly named among the four pillars.
Step 3: Place interfaces in their proper category.
An interface, as found in languages like Java or C#, is a language level construct, a contract of method signatures a class agrees to implement. It is a powerful tool that helps achieve abstraction and polymorphism, but it is a mechanism used to support the pillars, not one of the four foundational principles itself.
Step 4: Conclude.
The option that is not one of the core OOP principles is
\[ \boxed{\text{Interfaces}} \]