What is the primary unit of measurement for the amount of substance?

The primary unit of measurement for the amount of substance is the mole. The mole is a fundamental unit in the International System of Units (SI), and it is used to quantify the number of particles, such as atoms, molecules, ions, or other entities in a given sample. One mole is defined as (6.022 \times 10^{23}) particles, which is known as Avogadro's number. This allows chemists to count quantities of substances at a macroscopic scale by relating the number of particles to a measurable amount, bridging the gap between the atomic scale and everyday measurements.

The other choices are related but serve different purposes. The litre is a unit of volume often used to measure liquids, while the gram is a unit of mass used to weigh substances. The atom, though fundamental to the structure of matter, is not a unit of measurement but rather a building block of substance itself. Therefore, the mole is the most appropriate and widely accepted unit for expressing the amount of substance in chemical contexts.