Which of the following has the highest mass ?
(A) 1.2 mole of Mg
(B) $ 6.022\times \text{ }10{}^\text{2}{}^\text{1} $ atom of Ca
(C) 0.1 mole of phosphorus
(D) $ 6.022\times 10{}^\text{2}{}^\text{2} $ atom of Na

Hint: The mass of a molecule is measured in daltons and is called molecular mass (m). Because they contain various isotopes of an element, multiple molecules of the same chemical may have different molecular weights. The related quantity relative molecular mass is a unitless ratio of a molecule's mass to the unified atomic mass unit, as defined by IUPAC. The molecular mass and relative molecular mass are not the same as the molar mass, but they are linked.

Complete answer:
The mass of a sample of a chemical compound divided by the amount of material in that sample, measured in moles, is the molar mass of that compound in chemistry. In gram, it is the mass of 1 mole of the material, or $ 6.022\times {{10}^{23}} $ particles. The molar mass of a material is a bulk characteristic, not a molecular property. The molar mass is a weighted average of several different occurrences of the chemical, which might vary in mass due to isotopes.
The mole is simply a particle count. The particles being counted are usually chemically identical entities that are individually unique. A solution, for example, may include a number of dissolved molecules that are more or less independent of one another. The component particles of a solid, on the other hand, are stable and bonded in a lattice structure, yet they may be separated without losing their chemical identity. As a result, the solid is made up of a specific number of moles of these particles. Even in situations when the entire crystal is basically a single molecule, such as diamond, the mole is nevertheless employed to indicate the number of atoms bonded together rather than a count of numerous molecules.
Now using the formula
 $ \text{Number of moles=}\dfrac{\text{Given mass}}{\text{Molar mass}} $
We substitute the following options
(A) $ 1.2 \cdot \mathrm{mol} \cdot \mathrm{Mg} \equiv 1.2 \cdot \mathrm{mol} \times 24.31 \cdot \mathrm{g} \cdot \mathrm{mol}^{-1} \equiv 29.17 \cdot \mathrm{g} $
(B) $ 6.022 \times 10^{21} \cdot C \cdot $ atoms $ \equiv \dfrac{6.022 \times 10^{21} \cdot C \cdot \text { atoms }}{6.022 \times 10^{23} \cdot C \cdot \text { atoms } \cdot m o l^{-1}} \times 12.011 $ $ g \cdot m o l^{-1} \equiv 0.120 \cdot g $
 $ (C)\text{ }0.1\cdot mol\times \underbrace{31.00\cdot g\cdot mo{{l}^{-1}}}_{\text{molar mass of P}}=3.1\cdot g. $
(D) $ \dfrac{6.022 \times 10^{21} \cdot \text { sodium atoms }}{6.022 \times 10^{23} \cdot \text { sodium atoms } \cdot m o l^{-1}} \times \underbrace{22.99 \cdot g \cdot m o l^{-1}}_{\text {molar mass of } \mathrm{Na}}=0.230 \cdot g . $
Hence option A is correct.

Note:
The number of molecules, electrons, and other subatomic particles in a given volume of material is a dimensionless quantity that may be represented simply as a number and hence cannot be linked to a specific base unit.
The official mole is based on an out-of-date continuum notion of substance, which cannot logically apply to electrons or dissolved ions since there is no such thing as an electron or dissolved-ion substance. The SI thermodynamic mole is useless in analytical chemistry, and it might cost advanced economies money they don't have.