The compound most likely to dissolve in a polar solvent is HCl (hydrochloric acid).
HCl is a polar molecule due to the significant difference in electronegativity between hydrogen and chlorine, allowing it to interact favorably with polar solvents. In contrast, CH4 (methane), H2 (hydrogen gas), and O2 (oxygen gas) are nonpolar and are less likely to dissolve in polar solvents.