Find the average velocity if the takeoff velocity of an airplane on a runway is 300 km/hr with an acceleration of 1 m/s2 . A) 30 sec B) 41.7 sec C) 72.2 sec D) 83.33 sec

You have to decide what you want.  Your question says "Find the average velocity", but the choices you listed are all times, not velocities.

(300 km/hr) x (1 hr / 3600 sec) = 83.33 meters/sec

-- If the airplane starts from rest and accelerates uniformly at 1 m/s², it'll take 83.33 seconds to reach takeoff speed.

-- If it starts from rest and accelerates uniformly to 83.33 m/s, then its average speed is 41.67 m/s no matter what its acceleration may be.

i already answered it by 34/s