Among the following which is not isoelectronic with others

1. HF
2. H2O
3. NH3
4. ​CO

Dear student

Isoelectronic species have same number of  electrons.
The number of electrons in:
HF = 1+9 =10
H₂O = 2+8= 10
NH₃ = 7+3 =10
CO = 6+8 =14
Thus option (d) CO is not isoelectronic with the rest of the species.

Regards