Explanation:

A train 100 meter long completely passes a man walking in the same direction at 6 km/h in 5 seconds. (x = train)
5 = 100*18 / (x-6)*5
=> 5 = 1800 / (x-6)*5
=> 5 = 1800 / 5x-30
=> 25x - 150 = 1800
=> 25x = 1950
=> x = 78

A car travelling in the same direction in 6 seconds. (y = car)
6 = 100*18 / (x-y)*5
=> 6 = 1800 / (78-y)*5
=> 6 = 1800 / 390-5y
=> 2340 - 30 y = 1800
=> -30y = 1800 - 2340
=> -30y = -540
=> y = 18

Hence, option 1 is the correct answer.