Which of the following statements about acid-base indicators are true? A) Acid-base indicators are used to determine whether an unknown solution is an acid, base, or salt. B) Solution X + methyl orang

"Acid-base indicators are used to determine whether an unknown solution is an acid, base, or salt." Acid-base indicators change color depending on the pH of the solution, allowing us to classify a solution as acidic, basic, or neutral (which would result from a salt solution). This statement is true.

"Solution X + methyl orange = red, solution X is an acid." Methyl orange is an indicator that turns red in acidic solutions (typically below pH 3.1). Therefore, if solution X turns methyl orange red, it is an acid. This statement is true.

"Solution Y + methyl orange = yellow, solution Y is a base." Methyl orange turns yellow in basic solutions (typically above pH 4.4). Therefore, if solution Y turns methyl orange yellow, it is a base. This statement is true.

Since statements A, B, and C are all true, the option that encompasses all of them is the correct answer.

The final answer is $\boxed{\text{D) All of these}}$.