The linear molecule among the following is

Gaseous ${\mathrm{SnCl}}_2$ is ${\mathrm{sp}}^2$ hybridised and has one lone pair of electrons. Thus, it has angular shape. ${\mathrm{PbCl}}_2$ has also angular shape like ${\mathrm{SnCl}}_2$. ${\mathrm{SO}}_2$ is ${\mathrm{sp}}^2$ hybridized with a bent shape due to the presence of lone pair. In ${\mathrm{XeF}}_2$, xenon undergoes ${\mathrm{sp}}^3\mathrm{d}$ hybridisation and it has three lone pairs, hence the shape is linear.