Which of the following indicates that a chemical reaction has reached completion?

A chemical reaction is considered to have reached completion when there are observable changes that you can measure or identify, indicating that the reactants have been converted into products. The formation of a precipitate, a color change, or the evolution of gas are all reliable indicators of a change happening at the molecular level.

The formation of a precipitate suggests that a product is no longer soluble in the reaction mixture, implying that the reaction has progressed significantly. A color change can indicate a new substance has been formed with distinct properties compared to the reactants. Similarly, the evolution of gas denotes the production of gaseous byproducts, which is another clear sign that a reaction has taken place.

In contrast, while a change in temperature can occur during a reaction, it is not definitive evidence on its own that a reaction has reached completion. Additionally, if all components are still visible, it suggests that not all reactants have reacted, indicating the reaction may not be complete. Smell can also change during reactions but does not provide tangible evidence of completion. Therefore, the observable phenomena described in the correct choice represent solid and widely recognized indicators of a reaction that has reached completion.