ANS:A - the pages, belonging to the working set of the programs, are in main memory

For executing any program, CPU needs some data, it will 1st check main memory for that required data, if it present in main memory then CPU will use it, otherwise CPU retrieve the data (also called as page) from Secondary memory into primary memory and then execute the program.

When this page swapping (some pages transferred from 2nd memory to 1st and vice versa) occur in such an extent that CPU only do this swapping work and left all the necessary execution, then this state is called THRASHING.

So pages which are needed for execution, if already present in main memory (that is no need to swap from secondary memory) then thrashing can be avoided [that's why answer is A].

One best solution is -> Increase the size of main memory.