The correct option is C 18
To find after how many hours will the trains for the two stations, leave at the same time again next, we need to find the LCM.
Multiples of 2 = 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, ...... and so on
Multiples of 9 = 9, 18, 27, 36, 45, 54, 63, 72, 81, 90, ...... and so on
Hence, Least common multiple (LCM) of 2 and 9 is 18.
So after 18 hours, the trains for both the station will depart at the same time again next.