The key requirements are (1) avoid expensive licensing fees and (2) keep files locally. Linux best matches both. Quentin Docter explains that open source software is "freer than free," meaning the application is free and the source code is shared, and he explicitly lists "OSs such as Linux" as open source examples. That directly supports the licensing-cost goal. By contrast, Windows and macOS are commercial, licensed operating systems.
Chrome OS often works best when users rely heavily on cloud-based services and synchronization, which conflicts with the customer's preference that "all files be kept locally" (even though local storage exists, Chrome OS is commonly positioned around cloud workflows). Linux supports fully local file storage and gives the organization control over storage location, backup choices, and offline access.
The All-in-One guide reinforces the idea that some developers (including Linus Torvalds for Linux) made software "freely available for everyone," highlighting Linux as a no-licensing-fee option compared with commercial licensing models. Therefore, the best recommendation is Linux (A).