Step 1: Recall the memory hierarchy rule.
In the memory hierarchy, the closer storage sits to the CPU the faster (and smaller and costlier) it is.
Step 2: Place the slowest member.
The hard disk (HD) is secondary storage and is by far the slowest of the four.
Step 3: Place main memory next.
RAM is electronic and far quicker than a disk, so it sits above the HD in speed.
Step 4: Place the cache.
Cache memory is faster than RAM because it is small and sits very close to the processor.
Step 5: Place the fastest member.
CPU registers are inside the processor itself and are the fastest storage of all.
Step 6: Write the increasing-speed order.
From slowest to fastest the order is HD then RAM then Cache then CPU Register, which is option D.
\[ \boxed{\text{HD} \to \text{RAM} \to \text{Cache} \to \text{CPU Register}} \]