Alkanoic acids have higher boiling points than corresponding alkanols because they form stronger intramolecular hydrogen bonds. The carboxyl (-COOH) functional group in alkanoic acids allows two molecules to form two hydrogen bonds with each other, creating a stable cyclic dimer (a pair of molecules).

On the other hand, alkanols, which have a hydroxyl (-OH) group, can only form one hydrogen bond per molecule. However, more energy is required to break the stronger, more extensive hydrogen bonding in the alkanoic acid dimers, resulting in a higher boiling point.

The correct answer is option D.

A. alkanoic acids are more volatile than alkanols: This is incorrect. A higher boiling point means that a substance is less volatile, as it is harder to turn into a gas.

B. alkanols are more polar than alkanoic acids: This is incorrect. While both are polar, the ability to form more hydrogen bonds makes the overall intermolecular forces in alkanoic acids stronger and more extensive.

C. alkanoic acids are larger molecules: This is only true for certain comparisons (e.g., acetic acid has a higher mass than ethanol), but it's not the primary reason. The main reason is the difference in the strength and extent of hydrogen bonding, even for molecules of comparable molecular mass.