$O_2$  is isoelectronic with

Isoelectronic means same number of electrons in atom, molecules, formula unit or ion

Number of electron in $O_2^{-2}=2\times 8+2=18$

(a) Number of electron in $H_2=2\times 1=2$

(b) Number of electrons in $N_2=7\times 2=14$

(c) Number of electrons in $F_2=9\times 2=18$

(d) Number of electrons in $H{F}_2=1+9\times 2+1=20$

In given options, $F_2$  molecule contains same number of electrons (18).

$O_2$  is isoelectronic with $F_2$.