I am getting the data in drpdown list but while submitting it is giving error as
IllegalArgumnet Exception, So it needs String to store but getting object of SelectItems.
So please help me to resolve this problem, My code
I hope you realize that this is the wrong forum for your question as your problem has clearly nothing to do with RichFaces but with standard JSF.
Have a look at converters (f:converter) - they handle string / object conversion.