Answer : The correct option is, the number of electrons that fill the outer shell.
Explanation :
Element : It is a pure substance which is composed of atoms of similar elements.
The elements of similar characteristics have same number of valence electrons.
As we know that in the formation of compound the number of valence electrons participate in the bonding. So, the number of electrons that fill the outer shell determines the properties of elements.
Hence, the correct option is, the number of electrons that fill the outer shell.
