Which equation represents an equilibrium system? ?

Option B

Explanation:

A chemical reaction is said to be in equilibrium when the concentrations of reactants and products is the same - which means that the amount of reactants that become products is equal to the amount of products that become reactants. In other words, the reaction occurs with the same speed in both directions.

An equilibrium system is represented by a double arrow, which indicates that the reaction occurs in both directions. Among the choices given, we see that only the first two options have the double arrow: however, the first option is not a chemical reaction (it is just a change of state solid-gas), therefore the correct option is the second one.