(c) will be halvedCut off wavelength is given byλmin​=eVhc​, where h=Planck's constant c=speed of light e=charge on an electron V=potential difference applied to the tubeWhen potential difference (V) applied to the tube is doubled, cutoff wavelength (λmin​) is given by​λmin′​=e(2V)hc​⇒λmin​=2λmin​​​ Cuttoff wavelength does not depend on the separation between the filament and the target. Thus, cutoff wavelength will be halved if the the potential difference applied to the tube is doubled.