50 km is covered in 1 hour i.e 60 mins..Now, lets see for 40 km without stops.
50 km -> 60 mins
40 km -> ? x x= 48 mins.
But given is to cover 40 kms with stops it takes 1 hour i.e 60 mins
=> Time must have stopped for= 60 mins - 48 mins = 12 mins
Answer is (B)