What is the base SI unit for the amount of substance?

The base SI unit for the amount of substance is the mole. In scientific terms, the mole is defined as the amount of substance that contains as many entities (such as atoms, molecules, or ions) as there are in 12 grams of carbon-12. This unit is crucial in chemistry and physics for quantifying the number of particles in a substance, enabling scientists to relate mass to the number of entities at the atomic or molecular level.

The other choices represent different measurements and concepts. An atom refers specifically to the smallest unit of a chemical element and is not a unit of amount like the mole. The gram is a unit of mass, while the liter is a unit of volume. Neither the gram nor the liter can effectively represent the quantity of particles or entities in a substance, which is why the mole is the appropriate choice as it directly relates to the number of discrete particles.