B is the answer. The source of the light (i.e. a laser or LED or other light source) does not matter as much as the wavelength since the photoreceptors within the hair follicle only respond to certain wavelengths of light (668-684 nanometers is the optimal range as best we know). The light has to penetrate the scalp and likely takes 14-20 minutes to reach its target, so it has to be stationary for that time. Too much light therapy is inhibitory and too little is just ineffective, so every other day (or about 3 times per week) appears to be ideal.