Answer is: B. 2NaCl +H2SO4 ----> Na2SO4 + 2HCl.
According to principle of mass conservation, number of atoms must be equal on both side of balanced chemical reaction.
There are two sodium atoms, two chlorine atoms, two hydrogen atoms, one sulfur atom and four oxygen atoms on both side of balanced chemical reaction.
A) This reaction is not balanced, because number of oxygen atoms are not the same.
C) This reaction is not balanced, because number of oxygen and calcium atoms are not the same.
D) This reaction is not balanced, because number of sulfur atoms are not the same.