Answer
Drawing number $(1)$ is the best representation for that situation.
Work Step by Step
Let's use $NaOH$ as an example, when this base is added to the buffer, its acid $(HA)$ will try to neutralize the added base. And this is the reaction:
$NaOH(aq) + HA(aq) -- \gt Na^+(aq) + A^-(aq) + H_2O(l)$
Therefore, the concentration of $HA$ is decreasing, and $A^-$ is being produced.
The drawing with more $A^-$ and less $HA$ is the first one.
