Final answer:
Acetic acid and formaldehyde do not form a ring in their monomeric or dimeric forms due to hydrogen bonding. Therefore, the correct option is D.
Step-by-step explanation:
The molecules that do not form a ring either in their monomeric form or dimeric form due to hydrogen bonding are acetic acid and formaldehyde.
Acetic acid (CH3COOH) forms a dimer in the gas phase, where two acetic acid molecules are held together by strong intermolecular attractions.
Formaldehyde (CH2O) forms strong covalent bonds between the carbon and oxygen atoms, but it does not form a ring structure in its monomeric or dimeric form.