The minimum wavelength corresponds to the case when the electron transfers all of its energy to a photon. The electron has $25 \text{ keV}$ of energy. The wavelength of a photon is related to its energy by $E = 2 \pi \hbar c/\lambda$ . Therefore, the minimum wavelength is

$\begin{align*}\lambda_{min} &= \frac{2 \pi \hbar c}{25 \text{ keV}} \\&\approx \frac{2 \pi \cdot 200 \text{ MeV fm}}{...$
Therefore, answer (B) is correct.

Solution to 1992 Problem 80