The correct answer is D) 6. The mechanical advantage of a threefold purchase when rove to disadvantage and neglecting friction is 6. This is because in a threefold purchase, there are three parts of the rope supporting the load, which results in a mechanical advantage of 6 (3 parts x 2 = 6). The other options are incorrect because A) 3 is the mechanical advantage when the threefold purchase is rove to advantage, B) 4 is the mechanical advantage of a fourfold purchase, and C) 5 is not the correct mechanical advantage for a threefold purchase rove to disadvantage.