Speed in still water $$=6km/h$$
Speed in upstream $$=6-2 = 4km/h $$
Speed in downstream $$ =6+2=8 km/h$$
Distance =d
Time taken while going downstream =t hours
Time taken while going upstream = t+3 hours
Since the distance is same
$$8 imes t = 4 imes (t+3) = 4t +12$$
$$4t =12$$
$$t=3 hours$$
Distance = Speed in downstream $$ imes$$ Time taken while going downstream
$$=8 imes3$$
$$=24 km$$
Hence Option B is the correct answer.