The correct option is A Atomicity
Atomicity of an element is a measure of the total number of atoms present in a molecule. For example, an oxygen molecule (O2) consists of 2 oxygen atoms and hence its atomicity is 2. A formula is singular whereas formulae are plural, it is a short form of representing the molecules of an element or a compound. A molecule can be defined as the smallest unit of an element or a compound which exhibits all their properties and has an independent existence. They are divisible into atoms.
Compounds are pure substances formed by chemical combination of two or more elements in a definite proportion.