What is pH a measure of?

Answer: C — How acidic or basic a solution is

pH is a scale used to specify the acidity or basicity of an aqueous solution. It quantifies the concentration of hydrogen ions (H+) in the solution; a lower pH value indicates a higher acidity (more H+ ions), while a higher pH value corresponds to a more basic or alkaline solution (fewer H+ ions). This measurement is crucial in various fields, including chemistry, biology, environmental science, and many industrial applications, since the pH level can significantly impact chemical reactions, biological processes, and the behavior of substances in solutions. Other factors such as pressure, temperature, and density do not pertain to the definition of pH, thus differentiating the correct choice effectively.