I'm looking for a good 2d engine or library (that supports physics) for Android. I have a restriction - language should be C#. It's not a requirement to use Mono Droid, but it seems that it is an only way.
What can you suggest?
Farseer Physics Engine is probably the best 2D physics engine for .NET.
If XNA level library is enough you can try MonoGame.
If you want to go even more high level then try Unity.
I wouldn't be surprised if running physics via Mono on Android will turn out to be slow. Some people have issues in iOS. There should be improvements on this side in the new version of Farseer.
I would recommend to try some very similar sample first to check if it is possible to get desired performance.