Step 1: Concept
The spring constant ($k$) is inversely proportional to the length ($L$) of the spring ($k \propto 1/L$).
Step 2: Meaning
If a spring of length $L$ is cut into $n$ equal pieces, each piece has a length of $L/n$.
Step 3: Analysis
Since $k' \propto \frac{1}{L/n}$, the new spring constant $k' = n \times (\frac{1}{L}) \propto nk$.
Step 4: Conclusion
Hence, the force constant of each piece is $nk$.
Final Answer: (E)