Cross platform script errors

I'm getting namespace related errors such as:

Code (CSharp):

1. error CS0246: The type or namespace name 'BoomBox' could not be found (are you missing a using directive or an assembly reference?)

when changing the target platform from PC to Android (tried Web GL as well and that worked).

It looks like:

https://stackoverflow.com/questions/4764978/the-type-or-namespace-name-could-not-be-found

but, how can I find out the version of .Net that the android platform requires?