You must add a reference to assembly 'System.Xml.Linq, Version=4.0.0.0, culture = neutral,

I created a portable Library project in visual studio which contains a class and method for parsing XML Document. I created a Mono.Android project in the same Solution and referencing the Portable Library project by Mono.Android project. In my Android Project I am calling the method for Which I Written to Parse XML document. I am Passing XDocument object as a parameter to that method. But I am getting error : Error 1)The type 'System.Xml.Linq.XDocument' is defined in an assembly that is not referenced. You must add a reference to assembly 'System.Xml.Linq, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089'. C:\Users\pruthviraj\Documents\Visual Studio 2013\Projects\Xamarin\KernelTesting\MainActivity.cs 55 21 KernelTesting.
Please any one helpme. Thanks