OPTION A:
if we used fixed partitioning,variable partitioning then automatically we use the base and the limit registers.(we are performing address translation with the help of base and limit registers)
for paging and segmentation we use the pagetables and segment table for address translation .
these basically provide memory protection .if we would not have used address translation then only one program could be run .
OPTION B
we could use interrupt driven I/O instead of DMA
OPTION C
I think we need the atleat 2 modes for both single user and multiuser processing systems.
OPTION D
we can have a system in which all the pages of all the programs are in the main memory.so demad paging is not necessary.
so option A and C are correct.