The electronic configuration of an ion is determined by removing electrons from the outermost shell (4s) first, followed by the penultimate shell (3d).

1. Cobalt (Co, Z=27): The neutral atom configuration is $[Ar] 3d^7 4s^2$ .
2. Formation of Co³⁺: To form the +3 ion, 3 electrons are removed: 2 from the 4s orbital and 1 from the 3d orbital.

• $Co^{3+}$ configuration: $[Ar] 3d^6$.

Verification of other options:

• Ni³⁺ (Z=28): Neutral is $[Ar] 3d^8 4s^2$. Removing 3 electrons gives $[Ar] 3d^7$.
• Mn³⁺ (Z=25): Neutral is $[Ar] 3d^5 4s^2$. Removing 3 electrons gives $[Ar] 3d^4$.
• Fe³⁺ (Z=26): Neutral is $[Ar] 3d^6 4s^2$. Removing 3 electrons gives $[Ar] 3d^5$ (half-filled, stable) .