Cobalt 59 is a stable isotope and cobalt 60 is an unstable or radioactive isotope these two isotopes differ in the number of

Co-59 and Co-60 differ in the number of neutrons.

Explanation:

Co-59 and Co-60 are isotopes of one another. Isotopes differ from one another as they have different mass number but same atomic number. Mass number is the sum of the number of protons and neutrons. As they have the same atomic number they have same number of protons. They have different number of neutrons.  

Co has atomic number 27. So Co-59 and Co-60 have (59-27) or 32 and (60-27) or 33 number of neutrons respectively.  

b is the correct answer

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