Step 1: Understanding Moseley's law.
Moseley's law states that the square root of the frequency of the X-ray line is proportional to the atomic number \( z \) of the element. Mathematically:
\[
\sqrt{f} \propto (z - \text{constant})
\]
Step 2: Analyzing the options.
The graph should plot the square of the frequency (\( f \)) against the atomic number \( z \). The correct representation is option (B) \( f \) vs \( z \).
Step 3: Conclusion.
Therefore, the correct graph is option (B).
Final Answer:
\[
\boxed{\text{(B) } f \text{ vs } z}
\]