An iron nail floats in mercury, whereas it sinks in water because the density of iron is more than that of water and less than that of mercury (C). Density is the mass of a substance per unit volume. Iron has a higher density than water but a lower density than mercury. Therefore, when placed in mercury, which is denser than water, the iron nail displaces less mercury than its own weight, allowing it to float. In water, however, the iron nail displaces less water than its own weight, causing it to sink. Options (A) and (D) are incorrect because they do not explain the phenomenon of flotation based on density. Option (B) is also incorrect because it focuses solely on weight, disregarding the role of density. Therefore, the correct option is (C) The density of iron is more than that of water and less than that of mercury.