What does the group number of a elements in the a/b numbering system)
tell about valence electrons?

a. the a group number tells which period the valence electrons are
in
b. the a group number tells how much mass the elements have.
c. all elements in the same a group will have the same number of
valence electrons.
d. all elements in the a group will have the same valence electrons
as the b group.