To determine which feature is needed to make a programming language object-oriented, we need to understand the key principles of object-oriented programming (OOP).

Object-oriented programming is a programming paradigm that uses objects, which are instances of classes, to represent and manipulate data. OOP focuses on four main principles: encapsulation, inheritance, polymorphism, and abstraction. These principles help in organizing and structuring code for better modularity, reusability, and maintainability.

Now let's go through each option and explain why it is right or wrong:

A. Encapsulation: Encapsulation is the process of hiding the internal details and implementation of an object and providing access to only the necessary information. It allows for data protection and ensures that the object's internal state is accessed and modified through defined methods. Encapsulation is a fundamental principle of OOP, but it is not the only feature required to make a programming language object-oriented.

B. Inheritance: Inheritance is a mechanism that allows one class to inherit the properties and behaviors of another class. It promotes code reuse and allows for the creation of a hierarchy of classes. Inheritance is a key feature of OOP and plays a crucial role in achieving modularity and extensibility.

C. Polymorphism: Polymorphism refers to the ability of objects to take on multiple forms or have multiple behaviors. It allows for the use of a single interface to represent different types of objects. Polymorphism is another essential feature of OOP that enables code flexibility and modularity.

D. Both a and b: This option is partially correct. Encapsulation and inheritance are both features needed to make a programming language object-oriented. These two features work together to provide data hiding, code organization, and code reuse.

E. All of the above: This option is correct. All of the mentioned features (encapsulation, inheritance, and polymorphism) are needed to make a programming language object-oriented. These features work together to achieve the main principles and benefits of OOP.

Therefore, the correct answer is:

The Answer is: E. All of the above.