When using IWA through the Web Adaptor (IIS), the IIS server itself must be configured to deny anonymous access and allow Windows Authentication. Simply disabling anonymous access within the Portal settings is not enough.
From Esri's web-tier authentication guide:
"For IWA to function as expected, anonymous access must be disabled in IIS for the ArcGIS Web Adaptor, and Windows Authentication must be enabled. If IIS still allows anonymous access, users will bypass authentication prompts."
* Option B is irrelevant here-user types don't affect login behavior.
* Option C is a content sync feature, not related to authentication.
* Option D (Enable Windows Authentication) is correct only if it hasn't already been done, but the missing step is disabling anonymous access in IIS.
Reference:ArcGIS Enterprise - Configuring Web-Tier Authentication with IIS

For field operations, Vector Tile Services are highly efficient and suitable for basemaps. They offer fast rendering, smaller file sizes, and are easily scalable across various devices, including those with limited bandwidth or processing capabilities. Vector tiles also support dynamic styling and are ideal for offline use, which is often necessary for field crews working in areas with limited connectivity.
Cached map services (Option A) are raster-based and can be larger in size, making them less optimal for mobile use. Dynamic map services (Option C) require real-time data rendering, which can be resource- intensive and may not perform well in field conditions with limited connectivity.
Reference Source: ArcGIS Enterprise documentation on customizing basemaps