Solution:

We arrange memories by access speed (fastest → slowest). Typical hierarchy and reasoning:

1. Registers — located inside the CPU, they provide the fastest access (single CPU cycle or less).
2. Cache Memory — small, very fast memory between CPU and RAM (L1/L2/L3 levels). Slower than registers but much faster than main memory.
3. Random Access Memory (RAM) — main memory (DRAM); larger but slower than cache.
4. Hard Disk Drive (HDD) — mechanical storage with the highest latency and lowest access speed among the listed items.

✅ Correct order (fastest → slowest): Registers > Cache > RAM > HDD

✅ Corresponding option: (D) (D), (B), (C), (A)