The correct answer is B. 308 nm.
The operational wavelength of the 308 nm excimer laser and lamp is close to that of NB-UVB. The therapeutic benefit of the excimer laser for vitiligo has been investigated in multiple studies, and, overall, 20–50% of lesions achieve ≥75% repigmentation; the excimer lamp appears to have similar efficacy. Localized patches of vitiligo are treated one to three times weekly for an average of 24 to 48 sessions, with the repigmentation rate depending on the total number of sessions, not their frequency. Erythema and (rarely) blistering represent potential side effects.
The 532 nm (KTP), 595 nm (PDL), 755 nm (Alexandrite), and 1064 nm (ND:YAG) lasers have not been shown to be efficacious in the treatment of vitiligo.