Which of the following buffer solutions is used for precipitation...

In qualitative analysis, the cations of group III A (such as $\text{Fe}^{3+}$, $\text{Al}^{3+}$, and $\text{Cr}^{3+}$) are precipitated as their hydroxides.

The group reagent used for this purpose is a mixture of ammonium hydroxide ($\text{NH}_4\text{OH}$) and ammonium chloride ($\text{NH}_4\text{Cl}$).

The addition of $\text{NH}_4\text{Cl}$ suppresses the ionization of the weak base $\text{NH}_4\text{OH}$ due to the common ion effect. This maintains a low concentration of hydroxide ions ($\text{OH}^{-}$), which is just sufficient to exceed the solubility product ($K_{sp}$) of group III A hydroxides and precipitate them, while preventing the precipitation of hydroxides of subsequent groups which have higher $K_{sp}$ values.

Thus, the buffer solution used is $\text{NH}_4\text{OH} + \text{NH}_4\text{Cl}$ in water.

Answer: $\text{NH}_4\text{OH} + \text{NH}_4\text{Cl}$ in water.