If each of the following salts has solubility product $K_{sp}=1\times {10}^{-9},$ which of them is least soluble in water?

When a weak base solution $\left(50 \mathrm{mL}\right.$$\left.0.1 \mathrm{M} {\mathrm{NH}}_4\mathrm{OH}\right)$ is titrated with a strong acid $(0.1 \mathrm{M} \mathrm{HCl}),$ the $\mathrm{pH}$ of solution initially decreases fast and then decreases slowly till near equivalence point (as shown in the Which of the following statements is/are true?

A solution of a substance is titrated against a strong base (or acid), volume $\mathrm{V}$ of the strong base (or acid) is plotted against $\mathrm{pH}$ of the solution (as shown in the figure). The substance could be:

Select the correct graph(s) for the corresponding acid-base titration: