Which of the following operating systems is a free download with no evaluation period limitations?

Ubuntu 18.04 is a Linux-based operating system that is known for being open-source and freely available to users. This means anyone can download and install it without any cost, and there are no evaluation period limitations associated with it. Users have complete access to its full range of features without restrictions, which is a key characteristic of open-source software.

In contrast, Windows 10 Home typically requires the purchase of a license after a limited evaluation period. macOS is proprietary software developed by Apple and can only be installed on Apple hardware, which involves purchasing a Mac. iOS, similarly, is proprietary and specifically designed for Apple's mobile devices, also requiring purchase of those devices as well.

The nature of Ubuntu's licensing as a free and open-source operating system differentiates it from the other options that involve costs or restrictions, making it the only choice in this context.