If the pH of a basic solution changes from 12 to 9, which statement about the effect of hydroxide ions is most correct?

Correct answer: The basic effect has decreased about one thousand times

The pH scale is logarithmic, so a change of three pH units represents a thousand-fold change in hydrogen-ion concentration. Moving from pH 12 to pH 9 therefore greatly reduces the relative hydroxide-ion effect, although the solution remains basic because pH 9 is still above 7. Thus option A is the best statement; the solution is not acidic and hydroxide ions have not vanished.