What kind of software allows developers to access its source code and make modifications?

Open-source software is characterized by the fact that its source code is made available to the public. This transparency allows developers not only to view the code but also to modify it and distribute their changes. This collaborative approach fosters innovation and improvement, as a wider community can contribute to the development of the software. Open-source projects often encourage contributions from various developers, which can lead to enhanced functionality and security.

In contrast, closed-source software does not share its source code with users, thereby restricting modifications. Proprietary software also keeps its source code private, and users typically must purchase a license to use it, without the freedom to alter the code. License-required software encompasses a broader category that may include both open-source and proprietary types, depending on the licensing arrangement. Therefore, open-source distinctly allows modification and sharing, making it the correct choice in this context.