1. Write the electronic configuration of neutral iron

   Iron has atomic number $26$, so its electronic configuration is: $Fe: [Ar] \, 3d^6 4s^2$

2. Form the $Fe^{2+}$ ion

   When forming cations, electrons are removed first from the outermost shell.

   In transition metals, the $4s$ electrons are removed before the $3d$ electrons.

   So for $Fe^{2+}$: $Fe^{2+}: [Ar] \, 3d^6$

3. Count the number of $d$-electrons

   From $[Ar] \, 3d^6$, the number of $d$-electrons is: $6$

4. Check options

   • A: $4$ ❌
   • B: $5$ ❌
   • C: $6$ ✅
   • D: $3$ ❌

Hence, the correct option is C.