A software is as an open-source software if

The correct option is: Its source code is available to share, study and modify.

Open-source software (OSS) is defined by its licensing model, which grants users the legal right to access the software's underlying source code. Unlike proprietary software, where the code is hidden and restricted, open-source projects publish their source code so that anyone can inspect it, learn how it works, distribute copies to others, and make modifications to improve or customize the software.
Let's evaluate the other choices to understand why they are incorrect:

1. "The developer company is open 24x7": The operating hours of a development team or organization have no relevance to the legal licensing or accessibility of a software's source code.
2. "It can be downloaded from the internet": Many proprietary, closed-source programs (like free-to-play games or trial versions of commercial tools) can be downloaded from the internet, but their code remains locked.
3. "It is available free of cost": While most open-source software is free to download (often referred to as "freeware"), being free of cost does not automatically make it open-source. For instance, many closed-source applications are free to use but do not allow you to view or modify their source code. Conversely, open-source software can sometimes be sold or commercialized.

Therefore, the defining characteristic of open-source software is that its source code is openly available for sharing, studying, and modifying.