Answer:
C
Step-by-step explanation:
Every element within a group will have similar properties to every other element in that group (eg. Florine will have similar properties to Chlorine and Bromine, because all the elements are in group 17). One common property every element in a group will share is the number of valence electrons.
Valence electrons are the electrons in the outermost orbital of the atom, and these are the electrons that will be shared when an atom bonds to another atom.