Isoelectronic species are atoms or ions that have the same number of electrons. To determine which species are isoelectronic with CO₂, we need to calculate the total number of valence electrons in CO₂ and then for each given option.

Step 1: Calculate total valence electrons in CO₂.

Carbon (C) is in Group 14, so it has 4 valence electrons.
Oxygen (O) is in Group 16, so it has 6 valence electrons.

Total valence electrons in CO₂ = 4 (from C) + 2 × 6 (from 2 O atoms) = 4 + 12 = 16 valence electrons.

Step 2: Calculate total valence electrons for each option.

Option A) N₃⁻

Nitrogen (N) is in Group 15, so it has 5 valence electrons.
The ⁻ charge indicates an additional electron.

Total valence electrons in N₃⁻ = 3 × 5 (from 3 N atoms) + 1 (for the ⁻ charge) = 15 + 1 = 16 valence electrons.

Option B) CNO⁻

Carbon (C) has 4 valence electrons.
Nitrogen (N) has 5 valence electrons.
Oxygen (O) has 6 valence electrons.
The ⁻ charge indicates an additional electron.

Total valence electrons in CNO⁻ = 4 (from C) + 5 (from N) + 6 (from O) + 1 (for the ⁻ charge) = 16 valence electrons.

Option C) NCN²⁻

Nitrogen (N) has 5 valence electrons.
Carbon (C) has 4 valence electrons.
The ²⁻ charge indicates two additional electrons.

Total valence electrons in NCN²⁻ = 5 (from N) + 4 (from C) + 5 (from N) + 2 (for the ²⁻ charge) = 16 valence electrons.

Step 3: Compare the number of valence electrons.

All the species (N₃⁻, CNO⁻, and NCN²⁻) have 16 valence electrons, which is the same as CO₂.

Therefore, all the given options are isoelectronic with CO₂.

Correct Answer: (D)