Which of these never changes during a chemical change?

a. the arranhement of the atoms

b. the total number of molecules

c. the total number of atoms

d. the color of the reactants

B is not correct because new molecules are formed so numbers can change. D isn't correct because the color often changes, for example metallic iron and clear oxygen form red rust. The arrangement of the atoms changes, too so a isn't correct. C is cotrect