Which of these storage devices can act as a buffer between the CPU and the main memory ?

The processor operates far faster than main memory can supply data, which creates a bottleneck if the processor has to wait for every piece of data directly from main memory. Storage devices such as a hard disk, floppy disk, or flash drive are secondary storage media used for long-term data retention, and they are far too slow to sit directly between the processor and main memory during normal operation.

Cache memory solves this speed mismatch. It is a small, very fast block of memory placed between the CPU and main memory that temporarily holds frequently used data and instructions, allowing the processor to retrieve them quickly instead of waiting on slower main memory. Acting as this intermediary that smooths out the speed difference is exactly what is meant by a buffer between the CPU and main memory.

Remember cache memory by its position and purpose: it always sits between a faster component and a slower one, absorbing the speed difference so the faster component is not held up.