Answer
Choice D.
Work Step by Step
We measure the rms voltage, where $V_{rms}=\frac{V}{\sqrt{2}}$, and V is the amplitude (peak magnitude) of the voltage.
The amplitude V is $\sqrt{2}(1.5\;nV)$. The “potential difference between the two extremes” is twice this value, or 4.24 nV.
