A chemical reaction is a chemical process that involves the breaking and forming of chemical bonds. In the reaction, new compounds (Products) are formed with new properties different from those of the initial compounds (Reactants).
In the options, we see that the last three options are physical processes. The mixture of water and salt is a solution, but salt and water do not change their chemical structure. The last two options are a change of state, solid ice forms liquid water and gaseous carbon dioxide becomes a solid-state, they are just physical changes that do not affect the chemical structure.
Now, the first option is a chemical reaction, since a new bond is formed between sodium and chlorine, obtaining a new compound, salt. So the answer will be the first option.
Answer: Sodium and chlorine atoms bond to form salt molecules