The forces are balanced because an unbalanced force will cause the Ferris wheel to accelerate.
According to Newtons's first law of motion, a body will continue in its state of rest or uniform motion unless it is acted upon by an unbalanced force. According to the question, the Ferris wheel moves in a vertical circle at a constant speed.
Note that, the fact that the speed is constant implies that the forces are balanced because an unbalanced force will cause the Ferris wheel to accelerate.
