Answer
${\bf 0.87}\;\rm V$
Work Step by Step
We know, according to Ohm's law, that
$$\Delta V=IR$$
where $R=\rho L/A$ and $A=\pi r^2=\pi D^2/4$ where $D$ is the diameter.
$$\Delta V=I\dfrac{4\rho_{\rm copper} L}{\pi D^2}$$
Thus,
Plug the known;
$$\Delta V=(8)\dfrac{4(1.7\times 10^{-8})(20)}{\pi (2\times 10^{-3})^2}$$
$$\Delta V=\color{red}{\bf 0.866}\;\rm V$$
