All of these have multiple isotopes.
- Calcium 48
- Calcium 42
- Zinc 64
- Zinc 66
- Sodium 24
- Sodium 23 (stable most commonly occurring)
- Magnesium 24
- Magnesium 25
Magnesium has the least number of isotopes so if an answer had to be chosen it would be magnesium.
I did some research on the Internet and here's what I found:
(Note: these are STABLE ISOTOPES)
Mg has 3 isotopes, Mg-24, Mg-25 and Mg-26.
Zn has 5 isotopes, Zn-64, Zn-66, Zn-67, Zn-68 and Zn-70.
Na has a single stable isotope (i.e. Na-23).
Ca has 6 stable isotopes, Ca-40, Ca-42, Ca-43, Ca-44, Ca-46 and Ca-48.
So from this, Na does not have any other stable isotopes.