The main function of goblet cells is to secrete substances, such as mucin (glycoprotein). Single membrane-bound structures include rough endoplasmic reticulum and Golgi apparatus, so a goblet cell must have a lot of those to produce the glycoproteins.
i think the trap in this question is that we want the highest proportion of SINGLE membrane-bound structures, not the highest proportion of membrane bound structures in general
D) and E) have a really developed ER which is part of the endomembrane system BUT it is continuous! its not made up of a bunch of little membrane-bound structures
A) yes it has membrane projections increasing its surface but its still continuous
C) red blood cells are relatively small and lack organelles
B) goblet cells do have a really developed Golgi and ER as Darius says, also they stockpile a BUNCH of vesicles under its apical membrane, so it has a very high count of SINGLE membrane-bound structures