The defining characteristic of an object-oriented language is its set of core principles that structure programs around objects rather than actions. Option A correctly lists these key features: custom data types (classes) that bundle data and methods (encapsulation), information hiding, representing essential features without including background details (abstraction), the ability of objects to take on many forms (polymorphism), and creating new classes based on existing ones (inheritance). These concepts collectively distinguish the object-oriented paradigm from others like procedural or functional programming.

B. Portability is a feature of many languages, including non-object-oriented ones, and is not a unique or defining characteristic of OOP.

C. This describes static typing, a feature found in many paradigms (e.g., procedural C) and not present in all OOP languages (e.g., Python).

D. This describes procedural programming, which focuses on a sequence of instructions, contrasting with OOP's focus on objects and their interactions.

1. Weisfeld, M. (2019). The Object-Oriented Thought Process (5th ed.). Addison-Wesley Professional. Chapter 2, "How to Think in Terms of Objects," details encapsulation, inheritance, and polymorphism as fundamental OOP concepts.

2. MIT OpenCourseWare. (2016). 6.005 Software Construction, Reading 10: Abstract Data Types. Massachusetts Institute of Technology. Retrieved from ocw.mit.edu. This reading explains how abstract data types are the foundation for encapsulation and information hiding, central tenets of object-oriented design.

3. Stroustrup, B. (2013). The C++ Programming Language (4th ed.). Addison-Wesley. In Section 2.5, "Object-Oriented Programming," Stroustrup defines OOP through its support for data abstraction, encapsulation, and inheritance.