The purpose of the soft iron spheres mounted on arms on the binnacle is to compensate for __________.

Correct Answer: Option D — induced magnetism in the horizontal soft iron

Soft iron spheres, known as Kelvin spheres, are positioned on the binnacle to compensate for induced magnetism in the horizontal soft iron of the vessel. By adjusting their distance from the compass, they neutralize the horizontal magnetic forces that vary with the ship's heading, as detailed in Bowditch.