Atomicity of phosphorus is:
(a) one
(b) two
(c) three
(d) four

Hint: Atomicity is defined as the total number of atoms that constitute a molecule. In olden days atomicity was used in the same sense as valency. There are different types of atomicity.

Complete step by step answer:
Atomicity of an element is a measure of total number of atoms present in a molecule. Example, an oxygen molecule contains two oxygen atoms. Therefore, the atomicity of oxygen is 2. In case of ozone, the atomicity is 3.
We can determine the atomicity of any element. It is determined by the ratio of molecular mass of total number of atoms to atomic mass of an element.
For phosphorus,
Molecular mass = 120
Atomic mass = 30
Therefore, \[Atomicity=\dfrac{120}{30}=4\]

Thus the correct answer for the question is (d).

Additional Information:
i) Atomicity varies in allotropes. All the metals and carbon do not have a simple structure and also consist of a large and indefinite number of atoms bonded together. So their atomicity cannot be determined and is considered as 1.
ii) There are different types of atomicity. Monatomic means the molecule consists of only one atom. Diatomic means the molecule consists of two atoms. Triatomic means the molecule consists of 3 atoms and so on. Molecules with more than 3 atoms are called polyatomic. Phosphorus is polyatomic. Metals are considered as monatomic. Boron and Silicon are considered to be monatomic, because Boron has too many allotropes and Silicon forms a network lattice.
iii) Diphosphorus has a chemical formula ${{P}_{2}}$. The atomicity of this phosphorus is 2. Diphosphorus is obtained by heating white phosphorus of tetraphosphorus ${{P}_{4}}$ (atomicity of phosphorus in this is 4) at 1100K. ${{P}_{4}}$.

Note: There are chances that atomicity is confused with valency. Atomicity gives the number of atoms present in a molecule. Valency is the number of valence electrons present in the valence shell.