Electron configuration of Ni²⁺ by removing 4s electrons first Steps:

• Nickel (Ni) has atomic number 28, so neutral Ni has 28 electrons with configuration [Ar] 4s² 3d⁸ or 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁸.
• For transition metal cations like Ni²⁺, remove the two 4s electrons first due to higher energy of 4s orbital in ions.
• Ni²⁺ has 26 electrons, resulting in [Ar] 3d⁸.
• Full configuration: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁸.

Why D is correct:

• D follows the rule that in transition metal ions, 4s electrons are removed before 3d, yielding [Ar] 3d⁸ per Aufbau principle adjusted for ions.

Why the others are wrong:

• A: Omits 2p⁶ and 3s² subshells, resulting in incomplete and incorrect filling.
• B: Includes 4s², representing neutral Ni (28 electrons) instead of Ni²⁺ (26 electrons).
• C: Lacks 3d electrons and has 4s⁰, giving only 18 electrons like argon, not Ni²⁺.

Final answer: D