Isoelectronic Species:
Isoelectronic species are atoms, ions, or molecules that have the same number of electrons. This means that they have the same electronic configuration, even though they may be different elements or ions.

Ag Ion:
Ag ion refers to the silver ion, which is formed when a silver atom loses one electron. The electronic configuration of a silver ion is [Kr] 4d^10.

Zn2:
Zn2 refers to the zinc ion, which is formed when a zinc atom loses two electrons. The electronic configuration of a zinc ion is [Ar] 3d^10.

Cd2:
Cd2 refers to the cadmium ion, which is formed when a cadmium atom loses two electrons. The electronic configuration of a cadmium ion is [Kr] 4d^10.

Pd2:
Pd2 refers to the palladium ion, which is formed when a palladium atom loses two electrons. The electronic configuration of a palladium ion is [Kr] 4d^8.

Cu2:
Cu2 refers to the copper ion, which is formed when a copper atom loses two electrons. The electronic configuration of a copper ion is [Ar] 3d^9.

Explanation:
Ag ion is isoelectronic with Cd2 because both ions have the same number of electrons and the same electronic configuration. They both have a completely filled d subshell with 10 electrons. This is evident from the electronic configurations of Ag ion ([Kr] 4d^10) and Cd2 ([Kr] 4d^10).

On the other hand, Zn2, Pd2, and Cu2 have different electronic configurations. Zn2 has a completely filled d subshell with 10 electrons, similar to Ag ion and Cd2. However, Pd2 has an incomplete d subshell with 8 electrons, and Cu2 has an incomplete d subshell with 9 electrons.

Therefore, the correct answer is option 'B' (Cd2) because Ag ion is isoelectronic with Cd2, as both ions have the same number of electrons and the same electronic configuration of [Kr] 4d^10.