One of the customizations you can apply to your own Challenges is a custom basemap layer. This basemap layer will override whatever basemap MapRoulette users have defined as their default. This is useful in those cases where you know that a certain basemap is needed to successfully complete the tasks in your Challenge.
All the default global basemap layers from OSM’s editor layer index, the same ones that you would find in the iD editor, are available as presets. You can also add your own custom XYZ layer using the common pattern