If this is related to WoW, Fullscreen (Windowed) would be the option to use, Microsoft dropped the fullscreen support in DirectX 12, regarding to Windows 8.1 (via update) and Windows 10+ (natively available i.e dx12 available after fresh install of win10 or win11).
If you use Windows 11 you can ensure that Optimizations for windowed games is enabled by doing the following:
- Select the Start 🪟 button, then select Settings ⚙️.
- In Settings, select System > Display > Graphics > Change default graphics settings.
- Select the setting under Optimizations for windowed games to turn it On.
- Restart your game.
Unfortunately this can’t be enabled in Windows 10, I hope this helps.