Correct Answer - Option 3 : 15 sec

Given:

Speed = 72 km/h

Time to cross a man = 9 seconds

Length of platform = 120 m

Formula used:

Speed = distance/time

Calculation:

Speed = 72 × 5/18 = 20 m/s

The distance traveled to cross a man is equal to the length of the train.

∴ D = 20 × 9 = 180 m

Train's length = 180 m

The distance traveled to cross a platform is equal to the length of the train + the length of platform.

∴ D = 180 + 120 = 300 m

T = 300/20 = 15 sec

∴ The answer is 15 sec.