The formula equation that  shows   a  reversible  reaction  is

NH4Cl (s) ⇔ NH3 (g)  + HCl (g) ( answer  C)

            Explanation

• Reversible  reaction  is  a chemical reaction    where the reactants form products , where in turn react together to form reactant   back.

• The  reaction of  NH4Cl (s)⇔ NH3(g)  + HCl (g)  is a reversible reaction  since   NH4Cl  decompose to form NH3  and HCl which in  turn react together to form NH4Cl back .