Correct answer is 3. Explain why with all the steps

3. Which of the following is paramagnetic? ​

$$\begin{gathered} \left(1\right) {\mathrm{C}}_2^{2-} \\ \left(2\right) {\mathrm{Na}}_2{\mathrm{O}}_2 \\ \left(3\right) {\mathrm{NO}}_2 \\ \left(4\right) \mathrm{CO} \end{gathered}$$