The correct answer is Option 3. The assertion (A) is true because an iron ball floats on mercury due to the fact that mercury is denser than iron. This makes buoyancy greater than the gravity of the iron ball, causing it to float.
However, the reason (R) is false because it`s not accurate to state that the specific gravity of iron is more than that of mercury. Specific gravity is the ratio of the density of a substance to the density of a reference substance; equivalently, it is the ratio of the mass of a substance to the mass of a reference substance for the same given volume. The specific gravity of mercury is in fact greater than that of iron, which is why iron floats on mercury.
Therefore, while both the assertion and reason are related to the density relationship between iron and mercury, the reason doesn`t provide an accurate explanation for the assertion.