The mass of carbon-12 atom considered in the definition of a mole is

The correct option is "0.012 kg".

To understand why this is correct, we look at the standard definition of a mole in the International System of Units (SI).

Historically and under the SI system, one mole is defined as the amount of substance that contains as many elementary entities (such as atoms, molecules, or ions) as there are atoms in exactly 12 grams of carbon-12 ($C_{}^{12}$).

Since the kilogram (kg) is the base unit of mass in the SI system, we must express the reference mass of carbon-12 in kilograms instead of grams.
We know the conversion factor between grams and kilograms:
$1 g={10}^{-3} \mathrm{kg}$
Or:
$1 g=\frac{1}{1000} \mathrm{kg}$

Therefore, we can convert 12 grams to kilograms by calculating:
$12 g=12\times {10}^{-3} \mathrm{kg}=0.012 \mathrm{kg}$

Thus, the reference mass of carbon-12 considered in the definition of a mole is precisely 0.012 kg.