An excess of which of these ions tends to make a solution acidic?

The presence of hydronium ions (H3O+) in a solution is what makes it acidic. When an acid dissolves in water, it donates protons (H+) to water molecules, forming hydronium ions. The concentration of hydronium ions is a direct measure of the acidity of a solution; the higher the concentration, the more acidic the solution.

In contrast, chloride ions, hydroxyl ions, and sodium ions do not contribute to the acidity of a solution in the same way. Chloride is typically a neutral ion, hydroxyl ions (OH-) are associated with basic solutions, and sodium ions tend to be neutral as well. Consequently, it's the hydronium ions that play a crucial role in establishing an acidic environment in a solution, making them the correct choice in this context.