The term isoelectronic means same number of electrons.
Then you need to count the number of electrons in the given species NO₂⁻.
You have to count the number of electrons of N (nitrogen) atoms, the number of electrons of two O (oxygen) atoms, and the adjust for the negative charge of the ions.
The atomic number of N is 7, meaning that each atom has 7 protons and 7 electrons.
The atomic number of O is 8, meaning that each atom has 8 protons and 8 electrons.
The negative charge of the ion means that it has one extra electron, which is the responsible for the negative charge.
Therefore the total number of electrons of the ion NO₂⁻ is 7 + 2 (8) + 1 = 24.
Note that that 24 = 3 x 8, so 24 is the number of electrons in 3 atoms of oxygen.
The moleucle O₃ is an allotrope of the oxygen. (an allotrope is a a form of an element with different structure and physical properties in the same physical state).
Therefore, the allotrope of oxygen isoelectronic with NO₂⁻ is O₃ (ozone)