Final answer:
The substance with the lowest vapor pressure is propanoic acid (CH3CH2COOH).
Step-by-step explanation:
The substance with the lowest vapor pressure is propanoic acid (CH₃CH₂COOH). Vapor pressure is the pressure exerted by the vapor of a substance above its liquid or solid phase at a given temperature. Generally, a substance with a lower vapor pressure will have weaker intermolecular forces and will evaporate more easily.