Step 1: Picture the scenario described.
Imagine a process is given so little physical memory that almost every instruction it tries to execute needs a page that is not currently loaded. The OS keeps swapping pages in and out of disk just to keep up, and the CPU spends nearly all its time waiting on this swapping instead of doing useful work.
Step 2: Connect this to the underlying cause.
This happens when the number of frames allocated to a process falls below its actual working set, the set of pages it needs at that moment. Every access becomes a page fault, triggering another disk read and another swap, and the cycle repeats endlessly.
Step 3: Check why the other terms do not fit.
High CPU usage would mean the CPU is actually computing, here it is mostly idle or busy waiting on I/O. Network congestion has nothing to do with memory swapping. Memory fragmentation is about scattered free space, not about excessive paging activity.
Step 4: Name the phenomenon.
This condition of doing more paging than actual processing is specifically called
\[ \boxed{\text{Thrashing}} \]