How does this evidence indicate that the reaction has occurred between ions?

change of state of the reactants, e.g. precipitation.

Explanation:

The reaction of the ions can be checked by a number of methods. In a chemical reaction, the ions are in the acqeuos solution. This means that they are in solution, dissolved with charge. During the reaction (ionic) there is an attraction between the positive and negative ions. This makes a change in the state of the reactants in the products. In addition, the reaction can result in an immediate observable change such as the formation of a solid or precipitation.  

im pretty sure its b