The minimum wavelength of X-rays produced by an electron accelerated through a potential difference of $V$ volts is proportional to:

The energy of an electron accelerated through a potential $V$ is $E=eV$. This energy is converted into a photon of X-ray with maximum energy $h{\nu }_{max}$ or minimum wavelength ${\lambda }_{min}$.According to the Duane-Hunt Law:
$$eV=\frac{hc}{{\lambda }_{min}}$$
$${\lambda }_{min}=\frac{hc}{eV}$$
Since $h,c,$ and $e$ are constants:
$${\lambda }_{min}\propto \frac{1}{V}$$