Hi,

The key here is the acid catalysis - you can substitute the acetic anhydride with nucleophilic acyl substitution, liberate a carboxylic acid (because you have some acid around) and then do a fischer esterification resulting in 2 acetyl units being added (please let me know if this is unclear and I can draw a mechanism). It is actually mechanistically possible to do the two additions this way.

The crux of the matter for this question is that A B and C show the addition of acetyl units derived from acetic anhydride, while D only provides a formyl unit (one carbon instead of 2). This means that D is the most unlikely because it would require the breaking a C-C bond, which isn't really possible under these conditions.

I hope this helps!