Correct Answer:
A. Amount
The mole is the SI unit for the amount of substance. It quantifies the number of elementary entities (like atoms, molecules, or ions) present in a sample. One mole is defined as the amount of substance that contains Avogadro's number (approximately 6.022 x 10^23) of particles. While related to mass and volume for specific substances, the mole fundamentally measures the quantity of particles. Options B (Volume), C (Weight), and D (Length) are incorrect as they are distinct physical quantities measured by different SI units.