Answer
The stone will not land behind the mast.
Work Step by Step
Let's ignore air resistance.
If the stone is dropped from the top of a tall mast of a moving ship, its horizontal motion is not changed; it has the same horizontal speed that the ship has. From the viewpoint of someone on the moving ship, the stone falls in a vertical, straight line, landing exactly at the base of the mast.
Please see Figure 2.19 on page 34 for a similar example of a coin tossed upward in an airplane.
