The correct option is A 360 km
Distance travelled by the train at 80 km/hr in 30 min. = 40 km. This distance of 40 km is to be covered with a relative speed of
(90 - 80) = 10 km/hr.
Time taken to cover this distance = 4010 = 4 hr.
Hence, the trains will be together after 4 × 90=360 km from Delhi.