Which sulfide is soluble in $(NH_4)_2CO_3$?

Consider the following sequence of reactions:
$M^{2+}(aq.)$ $\xrightarrow{NH_4Cl(s) + (NH_4)_2CO_3 \, sol.} Q \downarrow$ $\xrightarrow[\text{Followed by addition of } (NH_4)_2C_2O_4]{CH_3COOH} R \downarrow$
Which of the following cations can form precipitate $Q$ but does not form precipitate $R$?

In which of the following is $NH_{3}$ not used?

Which compound is used to distinguish between $Fe^{2+}$ and $Fe^{3+}$ ions?

Which of the following does not give a precipitate with $CrO_4^{2-}$?

The group reagent for analytical group $IV$ is: