As of July 2021, World Topographic Map, which displays contour lines in Map Viewer Classic, was marked as deprecated. The replacement option does not display contours in Map Viewer Classic, though contours appear on the new Map Viewer. Unfortunately, the ArcGIS Web AppBuilder only runs maps through Map Viewer Classic, so map creators cannot create web map apps that display as shown in the new Map Viewer. Thus, there is currently no (non-deprecated) option to show a topographic map with contours on a web map app created through ArcGIS Web AppBuilder.
While the ArcGIS Web AppBuilder continues to utilize Map Viewer Classic, please provide an option for a topographic map that will display contour lines in this program.