option b and c are the reason i know are resolved by virtual memory concept.No idea of overlays..:p
for b-since in segmentation the memory is segmented so that there comes a complete function of a program in a segment. That way there would by less chances of page faults because it is expected that the instructions in a function would be most likely to be executed completely at a time.
for c-Since a function a segmented, no other process is allowed to use the segment block allocated to other. Thus it provides security.