Among the following, the diamagnetic compound is/ are:

The correct options are
B $N{a}_2{O}_2$
C $N_{2}O$
D $O_3$
If the number of electrons is even, then the molecule is usually diamagnetic - If all the electrons are paired.

A) $N{a}_2{O}_2$ ($11\times 2+8\times 2=38$ even number of electrons), so diamagnetic.

B) $O_3$ ($3\times 8=24$ even number of electrons), so diamagnetic.

C) $N_{2}O$ ($2\times 7+8=22$ even number of electrons), so diamagnetic.

D) $K{O}_2$ ($19+2\times 8=35$ odd number of electrons), so paramagnetic.

Hence options $A,B$ and $C$ are correct.