You can check the below link which is useful for to select AutoCAD version and .NET Framework.
Unless your plug-in uses AcCoreMgd.dll (Core Console), or .NET 4.0 Framework specific Types that are not available to .NET 3.5, you *should* be able to simply copy your source code files into a AutoCAD .NET 2010-2012 Wizard project and compile to .NET 3.5 without issue.
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