Among Fe²⁺, Fe³⁺, Cr²⁺ and Zn²⁺, identify the ion that:

• Shows a positive borax bead test.
• Has the highest ionisation enthalpy.

The borax bead test is used to identify transition metal ions based on characteristic coloured beads.

Fe³⁺ gives a characteristic yellow-brown bead in oxidising flame and is considered a positive borax bead test.

Ionisation enthalpy depends upon nuclear charge, electron configuration and stability.

Among the given ions:

• Fe²⁺ → 3d⁶
• Fe³⁺ → 3d⁵ (half-filled stable configuration)
• Cr²⁺ → 3d⁴
• Zn²⁺ → 3d¹⁰

Fe³⁺ possesses a particularly stable half-filled d⁵ configuration, resulting in high ionisation enthalpy.

Final Answer

Option (3) Fe³⁺