Arnold as the Default Renderer - Arnold for Cinema4d
To make Arnold your default renderer you must execute the following steps:
Create a new empty scene in Cinema 4D.
Go to the render settings and select Arnold Renderer.
Define your default settings (e.g. resolution, samples, etc).
Save your scene to the C4D install folder (where the Cinema 4D executable is located [c:\Program Files\Maxon Cinema 4D R25 on Windows, /Applications/Maxon Cinema 4D R25 on macOS]) using the name new.c4d.
Important:
Note that using a default scene as described above stores all render settings. This means that if a default value changes in a new Arnold release you will still see the old value in a new scene. In this case, you have to override new.c4d with the new settings.