Which statement about bases is true?

• A. A base produces hydrogen ions in water.
• B. Bases are acids.
• C. Bases increase the pH of a solution.
• D. A base produces hydroxide ions in water.

Answer: (D)

Bases in water are defined by producing hydroxide ions, which increases the solution’s pH. This hydroxide production is the defining behavior of a base in aqueous solutions, per the Arrhenius viewpoint. That makes the statement “a base produces hydroxide ions in water” true and the most precise description of how bases behave in solution.

The other claims don’t fit as well. Producing hydrogen ions in water describes acids, not bases. Saying bases are acids contradicts their basic definition. And while bases do tend to raise pH, the fundamental, universally true feature is generating OH− in solution, which directly explains why the pH increases.