$$ \text { The descending order of acidity among the following compounds is : } $$

Choose the correct answer from the options given below :

$$ \text { The strongest conjugate acid will result from : } $$

A D-aldotetrose on oxidation with concentrated $\mathrm{HNO}_3$ resulted in optically inactive dicarboxylic acid. The structure of the D-aldotetrose is :

Among $\mathrm{Fe}^{3+}, \mathrm{Pb}^{2+}, \mathrm{Cu}^{2+}$ and $\mathrm{Mn}^{2+}$, identify the one that gets precipitated out while passing $\mathrm{H}_2 \mathrm{~S}$ in presence of $\mathrm{NH}_4 \mathrm{OH}$ as group reagent. The highest possible oxidation state of the corresponding metal is