According to the law of conservation of mass, the total mass of reactants in a chemical reaction must equal the total mass of the products. In the reaction you provided:
\[ 2 \text{Na} + \text{Cl}_2 \rightarrow 2 \text{NaCl} \]
the mass of the reactants (sodium and chlorine) is 50 grams. Therefore, the total mass of sodium chloride (NaCl) formed when the reaction is complete will also be 50 grams.
The correct answer is:
D. exactly 50 grams