Final answer:
Water (H₂O) is the molecule most affected by diathermy (eddy currents) due to its highly polar nature, which causes its molecules to align and rotate in an electromagnetic field, generating heat.
Step-by-step explanation:
The type of molecules most affected by diathermy (eddy currents) based on the dipole effect are polar molecules. Among the options provided, water (H₂O) is the molecule most affected due to its polar nature, where the uneven distribution of charges creates a significant dipole moment. Polar molecules like water, when exposed to alternating electromagnetic fields, as in the case of diathermy treatments, have charges that align with the field, causing them to rotate and thus generate heat through molecular friction. In contrast, nonpolar molecules such as carbon dioxide (CO₂) and nitrogen dioxide (NO₂) do not have a significant dipole moment and are much less affected by diathermy. Ammonia (NH₃), while also a polar molecule, has a lower dipole moment compared to water, making water the most responsive to eddy currents induced by diathermy.