Most stable radio-isotope among the following –
O-16,O-17, O -18 are stable radioisotopes of oxygen.
C-12, C-13, are stable radioisotopes of carbon (not C-14).
P-31, is stable radioisotopes of phosphorus (not P-32).
1-125 is radioactive form of iodine (not stable).
O –18
C – 14
P – 32
I – 125