Memory Hierarchy

Step 1: Understanding the access speed of memories.
- **Registers** (D) are the fastest form of memory because they are directly connected to the CPU and used to store small amounts of data that the processor needs immediately.

- **Cache Memory** (B) is faster than RAM and stores frequently used data to speed up access time.

- **Random Access Memory (RAM)** (C) is slower than cache memory but faster than hard disk drives, and it is used as the main memory for running programs and data.

- **Hard Disk Drive (HDD)** (A) is the slowest memory type, as it is a mechanical storage device used to store large amounts of data permanently.

Step 2: Conclusion.
Thus, the correct order of memory access speeds from fastest to slowest is **(D), (C), (B), (A)**.