a
\(\begin{array}{l}
{F_x} = \frac{{ - dU}}{{dx}} = x\frac{{ - d}}{{dx}}\left( {\frac{{{x^2} - {z^2}}}{2}} \right) = - x\\
{F_Z} = \frac{{ - dU}}{{dz}} = \frac{{ - d}}{{dz}}\left( {\frac{{{x^2} - {z^2}}}{2}} \right) = z\\
\,\therefore \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\overrightarrow F = - x\hat i + z\hat k
\end{array}\)