The transition metal having highest 3rd ionisation enthalpy is:

Concept: 3rd Ionisation Enthalpy (IE₃)

IE₃ = energy to remove 3rd electron from M²⁺ ion.

M²⁺ configurations (4s electrons removed first):

| Element | M²⁺ Config | Electron removed | Reason for high IE₃ | |---------|------------|------------------|---------------------| | Cr | $3d^4$ | $3d^4 \to 3d^3$ | Gains half-filled | | Mn | $3d^5$ | $3d^5 \to 3d^4$ | Loses half-filled | | V | $3d^3$ | $3d^3 \to 3d^2$ | Normal | | Fe | $3d^6$ | $3d^6 \to 3d^5$ | Gains half-filled |

Mn²⁺ has the half-filled $3d^5$ configuration — removing the 3rd electron disrupts this extra-stable arrangement, requiring maximum energy.

Answer: Option (2) — Mn