Step 1: Understand the need.
The CPU works very fast, but main memory is slower. We want the memory that keeps the most used instructions ready so work goes faster.
Step 2: Recall cache memory.
Cache memory is a small but very fast memory placed inside or right next to the CPU. It stores the data and instructions used most often.
Step 3: Why cache speeds things up.
When the CPU needs something, it first checks the cache. If it is there, it gets it instantly, with no wait for slower memory.
Step 4: Check the other options.
A pen drive and a CD-ROM are removable and slow. A hard disk is large but also slow compared with the CPU. None of these sit close enough to feed the CPU quickly.
Step 5: Match the description.
Only cache memory is built to store frequently accessed instructions for faster processing.
Step 6: Conclude.
So the answer is option 1, cache memory.
\[ \boxed{\text{Cache Memory}} \]