Explanation:

Let length and speed of the train be x metre and v metre/second respectively.
Train crosses the post in 8 seconds
⇒x=8v ?(1)
Train crosses the platform in 20 seconds
⇒x+264=20v ?(2)
(2)−(1)?12v = 264

v = 264/12 = 22 m/s

=22 × 18/5 km/hr = 79.2 km/hr

Henece, option 1 is the correct answer.