Final answer:
To change an artboard named horizontal to landscape in UI design software like Adobe Illustrator, use the command Object > Artboards > Convert Orientation.
Step-by-step explanation:
You are working on designing a UI for the iPad Pro, which should be accessible in both portrait and landscape orientations. To change the artboard named horizontal to landscape, you will need to adjust its orientation. Within most UI design software, this typically involves altering the artboard settings. Looking at the provided options, the correct command to achieve this in Adobe Illustrator (assuming that's the software being used) would be option (d) Object > Artboards > Convert Orientation. This command will allow you to switch the orientation of the selected artboard from its current state to the desired orientation. The command implies that you are changing the orientation as opposed to just resizing or renaming the artboard. After selecting this option, the software may allow you to choose between portrait and landscape orientations, ensuring that you deliver a UI that is optimized for both viewing modes on the iPad Pro.