The question is incomplete, here is the complete question:
Which formula equation shows a reversible reaction?
A: 
B: 
C: 
D: 
<u>Answer:</u> The reversible reaction is 
<u>Explanation:</u>
Reversible reaction is defined as the reaction in which the products formed react together to give the reactants back. For general reversible chemical equation:

Irreversible reaction is defined as the reaction in which the reactants only lead to products and the reaction do not proceed back. For general irreversible chemical reaction:

For the given options:
The reaction having an equilibrium sign between the reactants and products is equation C.
Hence, the reversible reaction is 