What will be the mass of one 12C atom in g?

The correct option is 1.993 x (10)^-23 g.

To find the mass of a single carbon-12 (¹²C) atom, we can use the relationship between the molar mass of an element and Avogadro's number. By definition, one mole of carbon-12 has a mass of exactly 12 grams and contains Avogadro's number of atoms.

$$\text{Mass of one atom}=\frac{\text{Molar mass of carbon-12}}{\text{Avogadro's number}}$$

Here, the molar mass of carbon-12 is 12 g/mol, and Avogadro's number ($N_{\text{A}}$) is approximately $6.022\times {10}^{23}{\text{atoms mol}}^{-1}$.

Substituting these values into our formula gives:
$$\text{Mass of one}{}^{12}\text{C atom}=\frac{12\text{g/mol}}{6.022\times {10}^{23}\text{atoms/mol}}$$

Performing the division:
$$\text{Mass of one}{}^{12}\text{C atom}\approx 1.9927\times {10}^{-23}\text{g}$$

Rounding to three decimal places yields 1.993 x (10)^-23 g.