Final answer:
Benzene (C6H6) is the least soluble molecule in water compared to water (H2O), ammonia (NH3), and carbon dioxide (CO2), due to its nonpolar nature and inability to form hydrogen bonds with water.
Step-by-step explanation:
The molecule least soluble in water among H2O (water), NH3 (ammonia), CO2 (carbon dioxide), and C6H6 (benzene) is C6H6 (benzene). Solubility in water depends on the ability to form hydrogen bonds or interact through polarity, a characteristic that water itself (H2O), NH3, and CO2 possess to varying degrees, as they are polar or can form hydrogen bonds. Benzene, however, is a nonpolar hydrocarbon with a symmetrical arrangement of carbon and hydrogen atoms, which does not allow for hydrogen bonding with water, thus making it the least soluble in a polar solvent like water.