Which transition metal ion is most stable against oxidation to its +3…

The correct answer is Option (1) → $Mn^{2+}$

$Mn^{2+}$ is the transition metal ion most stable against oxidation to its $+3$ state.

Reasoning:

Stability depends on the electronic configuration of the ion formed after oxidation.

• $Mn^{2+}$ has configuration $3d^5$ (half-filled d-subshell).
• Half-filled subshells have extra stability due to symmetry and exchange energy.
• Oxidation to $Mn^{3+}$ would give $3d^4$, which is less stable.

Comparison:

• $Fe^{2+} \rightarrow 3d^6 \rightarrow$ can easily oxidize to $Fe^{3+}$ ($3d^5$ stable)
• $Cr^{2+} \rightarrow 3d^4 \rightarrow$ easily oxidizes to $Cr^{3+}$ ($3d^3$ stable)
• $Fe$ (metal) is not relevant to this specific ionic stability comparison

Therefore, $Mn^{2+}$ strongly resists oxidation to $+3$ because it already has the very stable half-filled $3d^5$ configuration.