I was told that in order to name an ester from a shortened structural formula, you drew out the full structural formula, find the ester link; draw a line which splits the link and then the half that contains the O-C bond belongs to the alcohol, meaning you count the carbons contained there; and then name it accordingly.
The half which contains the C=O bond is for the acid; and again, you count the carbons, name it accordingly.
With that in mind, I identified that the answer was B; the alcohol was propanol and the acid was ethanoic acid.
However, the answer given as A; i.e. the alcohol is Ethanol and the acid is butanoic acid.