Answer:- Sc^3⊕

Explanation:- 

Diamagnetic substances are those that have all of their electrons paired up in their atomic or molecular orbitals, resulting in no net magnetic moment. To determine which of the given ions (Cr²⁺, Fe³⁺, Cu²⁺, Sc³⁺) is diamagnetic, we need to look at their electron configurations.

a. Cr²⁺: The electron configuration of a neutral chromium atom is [Ar] 3d⁵ 4s¹. When it loses two electrons to become Cr²⁺, its electron configuration becomes [Ar] 3d⁴, which has unpaired electrons. Therefore, Cr²⁺ is paramagnetic, not diamagnetic.

b. Fe³⁺: The electron configuration of a neutral iron atom is [Ar] 3d⁶ 4s². When it loses three electrons to become Fe³⁺, its electron configuration becomes [Ar] 3d³, which has unpaired electrons. Therefore, Fe³⁺ is paramagnetic, not diamagnetic.

c. Cu²⁺: The electron configuration of a neutral copper atom is [Ar] 3d¹⁰ 4s¹. When it loses two electrons to become Cu²⁺, its electron configuration becomes [Ar] 3d⁹, which also has unpaired electrons. Therefore, Cu²⁺ is paramagnetic, not diamagnetic.

d. Sc³⁺: The electron configuration of a neutral scandium atom is [Ar] 3d⁰ 4s². When it loses three electrons to become Sc³⁺, its electron configuration becomes [Ar] 3d⁰, which means there are no unpaired electrons. Therefore, Sc³⁺ is diamagnetic.

So, the correct answer is option d, Sc³⁺, which is diamagnetic.