They are likely using an older version of RAS and RAS Mapper (not sure which one). Neither of those options given to you in the version you are running will give you the result you need for importing into Civil3D. You will likely notice that the file produced is way to small to be an elevation raster dataset. The latest version of RAS (6.4.1) gives another option to “Export Raster”, which is closer to what you want as it will produce an elevation raster dataset. However, I just tried it and the file is also not recognized by Civil3D.
I was able to import the raster produced by Mapper into ArcGIS and then export from Arc to a new Geotiff that will load into Civil3D, but that seems like a lot of steps.
You might try an older Mapper version. Or, the Arc route, though clunky, works.