Answer:
CO
Step-by-step explanation:
Henry's law constant reflects the solubility of a gas in water. The larger the kH, the more soluble is the gas. There is a rule that states that "like dissolves like", meaning polar is soluble in polar and nonpolar is soluble in nonpolar. Since water is polar, we have to consider the nature of these gases.
Xe nonpolar
Cl₂ nonpolar
CO polar
CO₂ nonpolar
CH₃CH₃ nonpolar
CO is the only polar gas, so it has the largest kH.