Which of the following options is not available as a basemap in ArcGIS Online?

In ArcGIS Online, Google Maps is not available as a basemap due to licensing and proprietary restrictions. Google Maps has its own terms of service that do not allow for its use as a map layer in other mapping services without specific agreements. This is in contrast to the other options listed, such as Bing Maps, OpenStreetMap, and Topographic Map, which are accessible and can be integrated into ArcGIS Online as basemaps without any licensing issues. Bing Maps is a Microsoft service that's compatible, OpenStreetMap is an open-access map provided by a community of mappers, and Topographic Maps are often used within GIS for their detailed terrain information.