Which quantity is needed to relate moles with the number of atoms?
A. Mass number
B. Atomic number
C. Volume
D. Avogadro number

Hint: The term mole, which is also pronounced as mol is a unit of standard used for measuring very large quantities of very small substances such as molecules, atoms, or some other specified particles.

Complete Solution :
- Avogadro's number is the number of units present in per mole of any substance (defined as its molecular weight in grams), and is equal to the value $6\times {{10}^{23}}$. The units mentioned above may be electrons, atoms, ions, or molecules, depending on the nature of the substance and the character of the reaction (if any).
-The value of the Avogadro constant was chosen such that the mass of one mole of a chemical compound, in grams, is numerically equal to the average mass of one molecule of the compound, measured in daltons (universal atomic mass units).
- Atomic no is defined as the total no of protons present in the nucleus of an atom or total no of electrons present in the orbits surrounding the nucleus.
- Mass no is actually the total no of protons and neutrons present in the nucleus of an atom.
- Volume is defined as the amount of space occupied by a substance, while mass is defined as the amount of matter that it contains. The amount of mass per unit of volume is called density.
- S.I unit of volume is the meter cube.
- So from the above the answer will be avogadro's number .
So, the correct answer is “Option A”.

Note: That long number is termed Avogadro's number after the first 19th-century Italian scientist Amadeo Avogadro. The mass of 1 mole of something in grams is the mass of that substance in mass units.