Step-by-step Solution:

Memory access speed increases as the storage unit gets physically closer to the CPU processing cores. The hierarchy from slowest to fastest is: 1. **Hard Drive**: Secondary, non-volatile storage (slowest). 2. **RAM**: Primary volatile main memory. 3. **Cache**: High-speed SRAM located close to the CPU to hold frequent instructions. 4. **CPU Registers**: The smallest and absolute fastest memory, located directly inside the CPU. Therefore, the correct increasing order is: Hard Drive \(\rightarrow\) RAM \(\rightarrow\) Cache \(\rightarrow\) CPU Registers.