ANS:D - can be caused by poor paging algorithms

Not all virtual memory systems are prone to thrashing. Thrashing is consequence of paging and not virtual memory.

The VAX-11 ISA that use a Paged Page Table in the virtual memory is the perfect example of Thrashing, not because it uses virtual memory but because it has to fetch the pages of the page table from the virtual memory causing thrashing.

The exact same VAX-11 ISA if used with 256 byte page size (instead of 512 byte) and 8-bit page address (instead of complete 22-bit available) has absolutely no thrashing whatsoever due to the page table being always loaded on the physical memory. It still has a 30-bit address bus for virtual addressing.