Usually, VREF is the IAS you want to achieve when crossing the runway threshold or very slightly before. As such, VREF does not change with the wind.It is a given value for a given temperature/weight/aircraft configuration. Conversely, VAPP = VREF + whatever wind/gust factor correction is the speed at which you fly the approach and it is that speed that changes as a function of the wind. As such, answer D is the proper answer.