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# Which will be the most stable among $Cu^{+},Fe^+,Fe^{2+}$ and $Fe^{3+}$

$(a)\;Fe^{2+}\qquad(b)\;Fe^+\qquad(c)\;Cu^+\qquad(d)\;Fe^{3+}$

$Fe^{3+}$ has $3d^5$ configuration (i.e) a half filled one and thus more stable.$Cu^+$ no doubt has $3d^{10}$ configuration but less stable because nuclear charge (29 protons) in Cu is not sufficient enough to hold a core of 18 electrons.
Hence (d) is the correct answer.