A buffer solution contains a weak acid $HA$ and $A^{ }$. When small quantity of $NaOH$ is added to keep pH as constant, which of the following reaction takes place?

$HA$$\to$ Weak acid and

$A^{ }$$\to$ Conjugate base

Now, $HA\rightleftharpoons H^{+}+A^{ }$ but $NaOH$ is a strong base.

Therefore, following reaction takes place

                                     $O{H}^{-}+HA\to H_{2}O+A^{-}$

Hence, the correct answer is option (C)  $HA+O{H}^{ }\rightleftharpoons H_{2}O+A^{ }$.