What is the final step in the preparation of a standard solution after transferring it to the volumetric flask?

The final step in the preparation of a standard solution involves stopping the flask and shaking it to ensure that the solute is completely and evenly mixed with the solvent. This step is crucial because it guarantees that the concentration of the solution is consistent throughout. If the flask is not shaken, areas with different concentrations may exist, leading to inaccuracies in any subsequent measurements or uses of the solution.

Once the solute is added to the volumetric flask, it is often necessary to add solvent (usually distilled water) up to the mark on the neck of the flask. Shaking the flask effectively disperses the solute throughout the solvent, establishing a uniform solution. This homogeneity is essential for accurate analytical work, as standard solutions are used for various quantitative analyses where precision is key.

The other choices do not represent critical final steps in the standard solution preparation. Allowing the solution to evaporate would actually change its concentration, while storing it in a particular environment doesn't directly contribute to the preparation process. Measuring temperature, although important in some contexts, is not a final step in the preparation of a standard solution itself. Thus, stopping and shaking the flask to mix it is the correct and crucial final action.