Answer
The answer is below.
Work Step by Step
When the object is going at terminal velocity in the x-direction, it still experiences a drag force. However, gravity is pulling the object in the y-direction, causing it to go faster and faster in the y-direction and less and less in the x-direction. As a result, we find: $x_{max}=\frac{mv_{x0}}{b}$.
We know that as time goes forward, the object goes further and further in the y-direction and less and less in the x-direction. Thus, the more in the x-direction the object is, the lower in the y-direction it will be. Therefore, the most realistic expression in this case is a function that has the shape of an upside down parabola.
