What is the oxidation number of carbon in $\mathrm{K}_2 \mathrm{C}_2 \mathrm{O}_4 ?$

What type of isomerism is exhibited by $\left[\mathrm{Cr}\left(\mathrm{H}_2 \mathrm{O}\right)_6\right] \mathrm{Cl}_3$ and $\left[\mathrm{Cr}\left(\mathrm{H}_2 \mathrm{O}\right)_5 \mathrm{Cl}\right] \mathrm{Cl}_2 \cdot \mathrm{H}_2 \mathrm{O}$

Which of the following groups exhibits (+)R effect?

Identify the product 'B' obtained in following reaction.

$\mathrm{R}-\mathrm{C} \equiv \mathrm{N} \xrightarrow[\mathrm{HCl}]{\mathrm{SnCl}_2}\mathrm{A} \xrightarrow{\mathrm{H}_3 \mathrm{O}^{+}} \mathrm{B}$