I think you may need to allow it to render longer. Are you limiting the render with a maximum time, or maximum samples limit under the Render Options?
i tried with both the options ... what's the difference with those ??? & how can I turn off both these options ????
This scene needs more time to render properly.
When you run a render it should give you an option for the resolution of the image you want to create. If you select a higher value in the preset drop-down it will take longer to render (hours) but will result in a high quality image.