The correct answer is B) two masts: with the mizzen stepped forward of the rudder post. A ketch is a type of sailing vessel with two masts, where the smaller of the two masts (the mizzen mast) is stepped forward of the rudder post. This configuration distinguishes a ketch from other two-masted sailing vessels, such as a yawl, where the mizzen mast is stepped abaft (behind) the rudder post. The other options are incorrect because A) describes a sloop, which has only one mast, C) describes a schooner, which has the foremast forward of the mainmast, and D) describes a yawl, not a ketch.