Concept:
Computer memory is organized in a hierarchy based on speed, cost and storage capacity. The closer a memory is to the CPU, the faster it is, but it usually has smaller storage capacity and higher cost.
The standard memory hierarchy is:
\[
\boxed{\text{Register} \rightarrow \text{Cache} \rightarrow \text{RAM} \rightarrow \text{Hard Disk}}
\]
Step 1: Understand the function of each memory.
• Registers are located inside the CPU and are the fastest memory.
• Cache Memory stores frequently used data for quick CPU access.
• RAM (Main Memory) temporarily stores programs and data currently in use.
• Hard Disk provides permanent storage but is much slower than RAM.
Step 2: Compare their speeds.
The speed decreases in the following order:
\[
\boxed{\text{Register} > \text{Cache} > \text{RAM} > \text{Hard Disk}}
\]
Similarly, storage capacity increases in the reverse order.
Step 3: Analyse the options.
• Hard Disk $\rightarrow$ Register $\rightarrow$ Cache $\rightarrow$ RAM -- Incorrect.
• RAM $\rightarrow$ Cache $\rightarrow$ Register $\rightarrow$ Hard Disk -- Incorrect.
• Register $\rightarrow$ Cache $\rightarrow$ RAM $\rightarrow$ Hard Disk -- Correct.
• RAM $\rightarrow$ Register $\rightarrow$ Cache $\rightarrow$ Hard Disk -- Incorrect.
Step 4: Final conclusion.
Hence, the correct memory hierarchy from fastest to slowest is:
\[
\boxed{\text{Register} \rightarrow \text{Cache} \rightarrow \text{RAM} \rightarrow \text{Hard Disk}}
\]