Which of the following species is a radical?

According to the question, radical is an exception case to the octet rule which occurs when there is an odd number of electrons in an atom.

In case of nitrogen oxide, it has $15$ Odd electrons (seven valency electrons of nitrogen and eight valency electrons of oxygen), so this species work as radical. 

In case of ${\mathrm{CO}}_2$, the total electrons are $16$ (even), in case of nitro ion, total valency electrons are $24$ (even), In case of cyanide ion, total valency electrons are $12$ (even).