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What is the definition of relative atomic mass,${{A}_{r}}$?
(A)$\left( \dfrac{average\text{ mass of naturally occurring atoms of an element}}{\text{mass of one atom of}{{\text{ }}^{12}}C} \right)\times 12$
(B)$\left( \dfrac{\text{average mass of naturally occurring atoms of an element}}{\text{mass of one atom of}{{\text{ }}^{12}}C\times 12} \right)$
(C)$\left( \dfrac{\text{average mass of naturally occurring atoms of an element}}{\text{mass of one atom of}{{\text{ }}^{12}}C} \right)$
(D)$\left( \dfrac{\text{mass of one atom of}{{\text{ }}^{12}}C}{\text{average mass of naturally occurring atoms of an element}} \right)$

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Hint: In relative atomic mass, masses are given in relation to the mass of $^{12}C$.Mass of Carbon- 12 is used as a reference to calculate the mass, in Relative atomic mass, isotopes are included in calculation.

Complete answer:
The relative atomic mass, ${{A}_{r}}$ is given by,
     $\left( \dfrac{average\text{ mass of naturally occurring atoms of an element}}{\text{mass of one atom of}{{\text{ }}^{12}}C} \right)\times 12$
The relative atomic mass is given by the average atomic mass , or mean of the atomic masses of all the atoms of an element found in a sampe and is then compared to the atomic mass of carbon-12.It is a dimension less quantity. According to IUPAC, relative atomic mass is the ratio of the average mass of the atom to the unified atomic mass unit, where unified atomic mass is the $\dfrac{1}{12}$ of the mass of an atom of $^{12}C$ in its ground state.
The IUPAC definition of the relative atomic mass is: An atomic weight of an element from a specified source is the ratio of the average mass per atom of the element to $\dfrac{1}{12}$ of the mass of an atom of $^{12}C$.
The concept of relative atomic mass was given by John Dalton and Thomas Thomas in between 1803 and 1805.

Note: When atomic mass and relative atomic mass is compared, we see that, Atomic mass is the sum of neutrons and protons present in an atom, however, Relative atomic mass is the average ratio of atoms, present in an element. In Atomic mass, Isotopes are not included in the calculation.