Identify the substrate ' $A$ ' in the following conversion.

$$ \text { A } \xrightarrow[\mathrm{H}_3 \mathrm{O}^{+}]{\mathrm{AlH}_4\left(-\mathrm{Bu}_2\right)} \text { Pent-3-enal } $$

Identify product ' $B$ ' in the following sequence of reaction.

$$ \text { Aniline } \xrightarrow[273 \mathrm{~K}]{\mathrm{NaNO}_2+\mathrm{HCl}} \mathrm{~A} \xrightarrow[\Delta]{\mathrm{H}_2 \mathrm{O}} \mathrm{~B}+\mathrm{N}_2 \uparrow $$

Identify the amine having highest $\mathrm{pK}_{\mathrm{b}}$ value.

Which from following compounds can be obtained by azo coupling reaction.