Which type of substance produces H+ ions in aqueous solutions?

The correct answer is that an acid produces H+ ions in aqueous solutions. By definition, acids are substances that, when dissolved in water, release hydrogen ions (H+) into the solution, increasing the concentration of H+ and thereby lowering the pH of the solution. This characteristic is what differentiates acids from other types of substances.

In contrast, bases are substances that can accept H+ ions or produce hydroxide ions (OH-) in solution, leading to a decrease in H+ concentration. Neutral substances do not significantly change the concentration of H+ or OH- ions in solution, maintaining a balance that results in a neutral pH of 7. Salts, which result from the neutralization reaction between an acid and a base, dissociate in water to produce ions but do not directly produce H+ ions unless they are derived from a reaction involving an acid.

Therefore, the defining characteristic of an acid in an aqueous solution is its ability to increase the concentration of H+ ions, making it the correct choice in this context.