Concept:
Nyquist sampling theorem:
\[
f_s \geq 2f_{max}
\]
Step 1: Given
\[
f_{max} = 4\,kHz
\]
Step 2: Apply formula
\[
f_s = 2 \times 4 = 8\,kHz
\]
Step 3: Interpretation
Sampling must be at least twice highest frequency.
Step 4: Avoid aliasing
Lower sampling leads to distortion.
Step 5: Conclusion
\[
\boxed{8\,kHz}
\]
Final Answer:
\[
\boxed{(B)}
\]