Answer
Drawing number $(3)$ represents the solution after the addition of a strong acid.
Work Step by Step
Let's use $HCl$ as an example, when this acid is added to the buffer, its base $A^-$ will try to neutralize the acid. And this is the reaction:
$HCl(aq) + A^-(aq) -- \gt Cl^-(aq) + HA(aq)$
Therefore, the concentration of $A^-$ is decreasing, and $[HA]$ is being produced.
The drawing with more $HA$ and less $A^-$ is the third one.
