In the interference phenomenon of light, the formula of fringe spacing is △x= L d λ, that is, the interference fringe spacing is proportional to the wavelength of incident light, proportional to the distance between two slits and the screen, and inversely proportional to the distance between two slits.

According to the meaning of the question, when the accelerating voltage decreases, the momentum of electrons decreases and the wavelength increases.

So choose B.