Which of the following pairs of ions,when their solutions are mixed,will result in precipitation?

$A$ very dilute acidic solution of $Cd^{2+}$ and $Ni^{2+}$ gives only yellow precipitate of $CdS$ on passing $H_2S$ gas. This is due to:

The solubility of $PbF_2$ in water at $25\,^{\circ}C$ is $\sim 10^{-3}\, M$. What is its solubility in $0.05\, M\, NaF$ solution? Assume the latter to be fully ionised.

If the $pH$ of a saturated solution of $Ba(OH)_2$ is $12$,what is the value of its solubility product constant $K_{sp}$?

The solubility product of $AgI$ at $25^{\circ} C$ is $1.0 \times 10^{-16} \ mol^2 \ L^{-2}$. What is the solubility of $AgI$ in $10^{-4} \ N \ KI$ at $25^{\circ} C$?

The solubility product of a salt having the general formula $MX_2$ in water is $4 \times 10^{-12}$. What is the concentration of $M^{2+}$ ions in the saturated aqueous solution of the salt?