Answer
Solution "A" 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 4.0 $\lt$ 6.0, the first solution (A) is more acidic.
