Cross platform application framework

I am planning to develop a cross platform application that will run on all operating systems mainly windows desktop, Mac desktop, android and Apple mobile devices. What are the cross platform alternatives that I should consider? I am very conversant with .Net and windows operating system. Some of the alternatives that I have researched are: QT=> A QT application needs to be written in C++ type code and it seems to have a long learning curve. wxWidgets, wxPython, Silverlight=> supported on both Windows and Mac operating systems.