Optical path =μ * Geo path
both have same Geo path but since for some lenge upper wave from S1 moves in medium of slab which has less refractive index than the entire medium.....hence S1C has optical path length less than S2C ...therefore lags
Chetan is right as refractive index of glass slab is less than the medium so the wave will have high speed in glass slab so it will lead.
@chetan_2020 I'm also telling that the wave from S1 lags but the key says it leads.
Clear my one concept:If optical path length is less than the wave lags or leads?
Obviously it will lead @sonu_2020.
Don't be confused , if the distance is less and speed is same then obviously less time will be consumed in less optical path.