Select all the species in which the central atom (the carbon atom, for $\mathrm{HCN}$ and $\mathrm{CH}{}_2\mathrm{O}$) is $sp$ hybridised.

Carbon in $\mathrm{CO}{}_2$ has two double bonds and carbon in $\mathrm{HCN}$ has one single and one triple bond, so each has two sigma bonds and no lone pair ($sp$). Sulfur in $\mathrm{SO}{}_2$ and carbon in $\mathrm{CH}{}_2\mathrm{O}$ each have three electron domains ($s{p}^2$).