Thank you very much for your fast reply.
As my sprite has multiple shapes and I was using Create CollisionPolygon2D Sibling, I was getting multiple collision shapes; one for each shape in the sprite. That led me to conclude that every shape in the sprite should have a single collision shape. I didn't thought that I can use multiple collision shapes for a single sprite shape, thanks for the hint !!!
In the meanwhile after posting this question, somehow I solved the problem. Here is what I have done (it may help somebody):
First of all, I downloaded a radial graph png file, & added it as background (temporarily)
Previously, my sprite had thick walls for the circular shape. So I started making the collision shape manually from inner side of the shape. I placed each point at 10 degrees. After reaching near the start point, I switched to outer side, reversed the direction & completed the whole shape. Later on I edited the start & end points such that there is no gap between them.
Next instead of using thick walls, I wanted to use circle with no (or little) stroke width. So I tried the same way, but now instead of marking the points at 10 degrees, I marked 20 degree points in one direction & other 20 degree marks in the revers direction. Now my collision shape was built, but after testing I noticed that the ball was passing through the shape.
So I reached to the conclusion that the collision shape must have a filled area to work properly. But then I tried to change thhe Build Mode from Solid to Segments & my problem was solved!!!