Correct answer choice is:
Velocity.
Explanation:
The velocity remains constant throughout the journey because the airplane can not reduce its velocity to maintain the balance in the air and to keep on moving.
In terms of plane cruising, the two central forces changing its velocity ahead are pull and thrust. At a fixed height, when the force of thrust equals the opposite force of resistance, then the plane will undergo consistent movement in one direction. This can be additionally defined by Newton's First Law.
