Explanation:

Linux is fundamentally an example of open-source software. This means its source code is freely available to the public, allowing anyone to view, modify, and distribute it under specific licenses. This collaborative development model fosters innovation and transparency. Freeware refers to software that is available at no cost but typically without access to its source code, limiting modification. Shareware is distributed with a trial period or limited functionality, requiring payment for full use. Complimentary software implies it's given for free, often as a bonus, but doesn't necessarily mean its source code is open. Therefore, 'Open sources software' is the most accurate and defining characteristic of Linux among the given options.