After wasting my one hour I came to know that only I had to include System.Windows.Interactivity as reference and which I had already done.
the issue was that Behaviors can not be declared in the Resource part of any control.
the moment I took the following
<i:Interaction.Behaviors>
</i:Interaction.Behaviors>
out of the resources then it worked fine.
So, conclusion only System.Windows.Interacivity is the only required.
Never declare that behavior part in the resources or style.