Acceleration refers to changes in velocity. Velocity refers to the direction something is traveling as well as the speed that something is traveling with.
Notice that question states that the airplane is traveling in a "straight line", meaning that the direction is not changing. The speed is stated to be a constant 609 m/s, meaning that the speed of the plane never changes.
Because the speed and the direction of the plane never changes, the velocity is constant. Because the velocity is constant, the acceleration is zero.