The correct answer is B) The Flinders bar. The Flinders bar is used to correct for the coefficient of deviation caused by the vertical component of the Earth's magnetic field inducing magnetism in vertical soft iron on a ship. The Flinders bar is a vertical bar of soft iron placed in the binnacle, which helps to cancel out the effects of this induced magnetism and improve the accuracy of the compass. The other options are incorrect because: A) Bar magnets in the binnacle would not correct for the effects of the vertical component of the Earth's magnetic field; C) Quadrantal soft iron spheres are used to correct for the coefficient of deviation caused by the horizontal component of the Earth's magnetic field; and D) The heeling magnet is used to correct for the effects of the ship's pitch and roll on the compass, not the vertical component of the Earth's magnetic field.