Answer
Solution $Y$ is more acidic.
Work Step by Step
Solutions with lower values of pH are more acidic.
That happens because: a solution with a greater $[H_3O^+]$ is considered more acidic, and since the pH equation has a negative logarithm, that relation is inverted.
$pH=−log[H_3O^+]$
Since 7.0 < 9.0, the first solution (A) is more acidic.
