Final answer:
The correct order of decreasing boiling points for the given substances is H₂ > He > Ar.
Step-by-step explanation:
The correct order of decreasing boiling points for the given substances is option c. H₂ > He > Ar.
The boiling points of substances depend on the strength and types of intermolecular forces present in them. The stronger the intermolecular forces, the higher the boiling point.
In this case, hydrogen (H₂) has the highest boiling point because it forms strong hydrogen bonds between its molecules. Argon (Ar) has the lowest boiling point because it is a noble gas and does not form intermolecular bonds. Helium (He) has a slightly higher boiling point than argon due to its smaller atomic size, which allows for weaker intermolecular forces.