Dimerisation of HOOC-COOH in non polar solvents
I believe your answer key is wrong on this one. Carboxylic acids like acetic acid (a) are well known to form doubly hydrogen bonded dimers in which each the carbonyl group on one acid is hydrogen bonded to the OH of the other.



For more explanation and structures, see https://www.chemguide.co.uk/organicprops/acids/background.html.

Oxalic acid, being a carboxylic acid, can participate in this kind of intermolecular association, but the result would be a polymeric structure, rather than a simple dimer.

The alcohols can hydrogen bond to each other only singly - too weakly to form stable dimers.

Kudos to you for having the right instincts to challenge your answer key an probe more deeply!