Answer:
I - 131 is the correct answer.
I - 131 is the only naturally occuring radioactive isotope among these four options.
Explanation:
Np - 239 - Neptunium is also considered as an artificial element,as only trace amounts of it are found in nature.
And Np - 239 is not a naturally occuring radioactive isotope but it is synthesized artificially.
No - This is an artificial element.That means its not found in nature. Hence all its isotopes are also synthesized artificially.
Cm - This is also an artificial element and all its isotopes are also synthesized artificially.
Np - 239,No - 254,Cm - 242 are radioactive isotopes but they are not naturally occuring radioactive isotopes. But these three are artificially made radioactive isotopes.
And Iodine - 131 is the only naturally occuring radioisotope among these.
