The correct answer is C) two depth readings. When a fathometer (depth sounder) is operated over a muddy or soft bottom, it can sometimes pick up two depth readings - one for the actual bottom and another for the layer of mud or soft sediment on top of the harder bottom. This is because the fathometer is sending out sound waves that can be reflected off both the mud/sediment layer and the harder bottom beneath it, resulting in two separate depth readings. The other answer choices are incorrect because A) a shallow depth reading would not account for the two-layer effect, B) no depth reading is unlikely as the fathometer should still detect at least one of the layers, and D) a zero depth reading would be inaccurate as the depth would still be measured, even if it is reflecting off the mud/sediment layer.