Consider the following pairs of ions
A) Sc⁺³ and Ti⁺⁴ B) Mn⁺² and Fe⁺²
C) Fe⁺² and Co⁺³ D) Cu⁺ and Zn⁺²
Among these pairs of ions, isoelectronic pairs would include

• Isoelectronic species refers to two atoms, ions  or molecules that have the  same number of valence electrons .(or)
• Atoms or ions with the same electronic configurations are said to be isoelectronic to each other 

A) Sc⁺³ and Ti⁺⁴ 

₂₁Sc⁺³ [Ar] ;  ₂₂Ti⁺⁴ [Ar]

 B) Mn⁺² and Fe⁺²

₂₅Mn⁺² [Ar] 3d⁵ ;  ₂₆Fe⁺² [Ar]3d⁶

C) Fe⁺² and Co⁺³

 ₂₆Fe⁺² [Ar]3d⁶ ; ₂₇Co⁺³[Ar]3d⁶

 D) Cu⁺ and Zn⁺²

₂₉Cu⁺ [Ar]3d¹⁰ ; Zn⁺² [Ar]3d¹⁰