What can decrease the pH of a solution?

The correct answer highlights that increasing hydrogen-ions in a solution will decrease the pH. pH is a measure of the acidity or alkalinity of a solution, and it is inversely related to the concentration of hydrogen ions. When hydrogen ion concentration increases, the solution becomes more acidic, resulting in a lower pH value.

In a practical scenario, when acids dissolve in water, they release hydrogen ions, leading to an increase in their concentration. This action decreases the pH, making the solution more acidic. Understanding this principle is essential in fields like chemistry and biochemistry, where pH plays a critical role in various reactions and processes.

Other options do not effectively decrease pH. For instance, decreasing hydrogen ions would increase pH, moving the solution towards a more neutral or basic state. Increasing temperature can alter the dissociation of certain substances but does not directly correlate with a decrease in pH without specifying the nature of the substances involved. Using a buffer solution typically helps maintain pH levels rather than decrease them, as buffers are designed to resist changes in pH when acids or bases are added. Thus, increasing hydrogen-ions is the clear and straightforward choice for decreasing pH.