Arrange O₂, O₂⁺, O₂⁻ and O₂²⁻ in order of increasing bond length. Which of them are paramagnetic?

Step 1. Bond orders. $\text{O}_2^+$: $\dfrac{10 - 5}{2} = 2.5$. $\text{O}_2$: $2$. $\text{O}_2^-$: $\dfrac{10 - 7}{2} = 1.5$. $\text{O}_2^{2-}$: $\dfrac{10 - 8}{2} = 1$.

Step 2. Lengths. The lower the bond order, the longer the bond: $\text{O}_2^+ < \text{O}_2 < \text{O}_2^- < \text{O}_2^{2-}$.

Step 3. Magnetism. The $\pi^*$ orbitals hold $1$, $2$, $3$ and $4$ electrons. Only with $4$ are they all paired.