Which of the following compounds on reaction with ammonical silver nitrate solution forms precipitate of silver?

Identify $A$ in the following reaction.

$$A+\mathrm{CH}_3 \mathrm{MgBr} \xrightarrow{\text { Ether }} \text { complex }\xrightarrow{\mathrm{H}_3 \mathrm{O}^{+}}\left(\mathrm{CH}_3\right)_3 \mathrm{C}-\mathrm{OH}$$

Which of the following is a Stephen reaction?

Which of the following compound is highly reactive towards HCN?