Answer: Option (A) is the correct answer.
Step-by-step explanation:
A pH strip is a strip which helps in determining whether a solution is acidic or basic in nature.
That is, if a solution is acidic in nature then a pH strip will change from blue to red. Whereas when a solution is basic in nature then a pH strip will change from red to blue.
Thus, we can conclude that statement molecules in the paper react differently with hydronium and hydroxide ions, best describes how a pH test strip works.