In the iso electronic species the ionic radii (AO) of N3−, O−2, F− are respectively given by
1.71, 1.40, 1.36
N3−, O−2, F− have 8 electrons in the outermost shell,so they are isoelectronic.
And we know that more the negative charge, larger the size among iso electronic species.
Hence , C is the correct option.