Thrashing is a condition when excessive paging are taken place .
For Option a : If they say page size is too small , Then for a process running It can bring more number of pages in memory . SO you dont have excessive paging . You have majority of pages in memory
For optio D and C : FIFO and LRU cant solve Thrashing problem . I think priority based algorithm can solve this problem . Hence it is wrong
For option b : There are many user connect to the system . mean many process are simultaneously running in the system.
For more process less number of pages for each process would be in memory . So now for majority of new page request by a process it has to replace its exists page . Hence the process of paging is done every now and then .
So b is answer