The density of an iron is $7800kg/{m^3}$. Its relative density will be:
A. $7.8$
B. $3.9$
C. $15.6$
D. $0.78$

Hint:For solving this question, we need to understand the concept of relative density. We know that density of any material can be defined as the amount of mass in a unit volume of matter. Every material has a different value of density. The relative density of a material is defined as the ratio of density of the material to the density of a standard liquid which is water.

Complete step by step answer:
Here, we are given that The density of an iron is $7800kg/{m^3}$.
As per the definition of relative density, we will find the relative density of the given material by dividing its density to the density of water.
We know that the density of water is $1000kg/{m^3}$.
Therefore,
\[
Relative{\text{ }}density{\text{ }}of{\text{ }}iron = \dfrac{{density{\text{ }}of{\text{ }}iron{\text{ }}}}{{density{\text{ }}of{\text{ }}liquid}} \\
Relative{\text{ }}density{\text{ }}of{\text{ }}iron = \dfrac{{7800kg/{m^3}}}{{1000kg/{m^3}}} \\
Relative{\text{ }}density{\text{ }}of{\text{ }}iron = 7.8\\
\]
Thus, the relative density of iron is 7.8.

Hence, option A is the right answer.

Note: We have determined the relative density of iron which is unit less. This is because it is the ratio of the density of two different materials with the same unit. Relative density is also known as the specific gravity of the material. The specific gravity of a substance will help us to know if it will float or sink, it gives us the knowledge about relative mass or relative density. If the specific gravity of a substance is less than one then it will float and if it is greater than one it will sink.