Answer
Calcium - 40 is the most stable and most likely to be found occurring naturally
Explanation
Nuclear stability means that the nucleus of the element is stable enough to not decay with the passing of time, this decay is known as radioactivity. When it comes to Calcium, we have many isotopes of this element, but only a few have such a long half-life that we can consider them to be stable, and Calcium - 40 is the most stable and common of them.
