The correct option is C.
Elements that has the same properties are usually grouped in the same group. The similarities of the elements in a group usually arise from the fact that they have the same number of electrons in their outermost shells. For instance, all elements in group 1 have one electron in their outermost shells and all elements in group 7 has seven electrons in their outermost shells. Thus, the number of a group signifies the number of electrons the elements in the group have in their outermost shells.