d. The frictional resistance to movement of nucleic acid molecules through the pores of a gel increases with size more rapidly than does the charge (which increases linearly, since each nucleotide contributes an additional negative charge). Consequently, the ratio of electric force to frictional force is greater for smaller molecules.
Most of you did well on this question. A few of you were confused by part (b): you thought that large and small molecules have equal charge. I suspect that you were thinking of charge "density", the ratio of charge to molecular size. Charge density would be the same for small and large nucleic acids, but total charge is determined by the number of nucleotides and is therefore directly proportional to molecular size.
A few of you didn't provide an answer to the last question; you simply restated the question in different words.