The correct answer is:
B. 2Na + Cl2 --> 2NaCl
In fact, this is the only reaction among the choices where the number of atoms of each element is conserved. In fact, we see that on the left we have 2 atoms of sodium and 2 atoms of chlorine, and on the right we have two molecules of NaCl, which means that we still have 2 atoms of sodium (Na) and 2 atoms of chlorine (Cl).
