I want to migrate a SharePoint 2010 .WSP (developed in C#) solution in Farm Basis to SharePoint Online. Please advice how to do this. I know in SP online Site Setting --> Solution we can upload the wsp and activate it only for sandbox wsp but not sure how to move Farm based wsp from SP 2010 to SP Online?
You can't do this easily. Farm solutions and sandbox solutions with custom code aren't supported in SharePoint Online. Generally you must rewrite your application as SharePoint Add-in (SharePoint-hosted or Provider-hosted). You should read this guide about transformation process.