What Is an Open System

What is an open system?

A) Any proprietary software licensed under exclusive legal right of the copyright holder.
B) Any software whose source code is made available free for any third party to review and modify.
C) Contains instructions written by a programmer specifying the actions to be performed by computer software.
D) Nonproprietary hardware and software based on publicly known standards that allow third parties to create add-on products to plug into or interoperate.