The source book printed no answer; this one was worked out and checked — see the explanation.
Explanation
¹²⁹I, ⁵⁷Fe and ¹²¹Sb all have low-lying nuclear excited states used in Mössbauer spectroscopy. ⁵⁷Co is only the radioactive source that decays to the ⁵⁷Fe excited state, not a Mössbauer nucleus itself.