The specific gravity of a substance is defined as the ratio of its density to the density of water. In this case, the specific gravity of cast iron is provided as 7.20.

To calculate the density of the cast iron, we need to use the specific gravity and the density of water. The density of water is approximately 1000 kg/m3.

So, to find the density of the cast iron, we can multiply the specific gravity by the density of water.

7.20 * 1000 = 7200 kg/m3

Therefore, the correct answer is option 4 - 7200 kg/m3.

It`s important to note that the units for density are in kilograms per cubic meter (kg/m3) and not kilograms per square meter (kg/ma) or kilograms per square meter (kg/m3).