The ArcGIS Online base map option includes, by default, the Streets, Topographical, and Imagery base map services:

The ArcGIS Online services are provided in the Web Mercator projected coordinate system (WKID 102100).

You may configure which services are included in the ArcGIS Online group on a site-by-site basis. To do this, edit the ESRI Base Maps list so that the items with a Group Name of "ArcGIS Online" have Urls that specify the desired services. You can also control the label of the base map radio buttons by editing the Display Name of these items.