Answer is A
With a computer, thrashing or disk thrashing describes a condition when a page is moved to hard sick from main memory inorder to bring in another page and the moved out page has to come to main memory again which causes another important page to be swapped out and this causes a chain of page swaps and thus slowing down the system enormously. Thrashing occurs when the system does not have enough memory, the system swap file is not properly configured, too much is running at the same time, or has low system resources.
Even if users are more we cannot say it can directly cause thrashing as they need not be using a lot of memory. (Increasing the level of multitasking is a possible cause of thrashing as it causes more no. of processes to require pages in memory simultaneously)
A is a better answer here because if page size is large there will be lesser number page swaps -- say if page size is 4KB, and there are 100 page swaps, if page size is made 8KB, we expect the number of page swaps to be reduced to 50 (this can be any value from 0-100, but expected value is 50).