According to Henry Moseley _______________is the fundamental property of the elements.
A. Atomic mass
B. Atomic number
C. Valency
D. Electronic configuration

Hint: Henry Moseley gave a long form of periodic table which was an improvement on Mendeleev’s periodic table in which the elements were arranged in the order of increasing atomic mass of the elements.

Step-by-Step Solution-
Henry Moseley gave the modern periodic table based on the increasing atomic number of the elements. It was because the previous periodic table given by Mendeleev was based on atomic masses and had many defects like-
Order of increasing atomic weight was not strictly followed in the arrangement of elements in the periodic table.
Similar elements were placed in different groups and different elements were placed in the same group.
It didn’t explain the cause of periodicity.
According to Henry Moseley, Atomic number is the fundamental property of the elements because elements are made up of atoms .An atom contains neutrons and protons at the center with electrons revolving around it and Atomic number is the number of protons or electrons present in an atom of an element. So the physical and chemical properties of elements are periodic functions of their atomic number. If the atoms are arranged in the order of their increasing atomic number then after regular interval similar properties are repeated.

Answer-Hence correct answer is B.

Note:
Atomic mass is the sum of the number of protons and neutrons. It is not necessary that all atoms of an element have the same number of neutrons; the number of neutrons may differ.
Valency is the capacity of an atom to combine with another atom. It is always a whole number.
Electronic configuration is the arrangement of electrons in the orbitals of an atom.