$1-$chlorobutane on treatment with alcoholic potash forms

Which among the following compounds will undergo the fastest $S_{N}2$ reaction?

An organic compound $A$ $(C_4H_9Cl)$ on reaction with $Na$ in diethyl ether gives a hydrocarbon,which on monochlorination gives only one chloro derivative. $A$ is:

The major product of the following reaction is:

In the following reaction,the product $(Y)$ will be:

$R-X + NaNO_2 \to P + NaX$
$R-X + AgNO_2 \to Q + AgX$
$P$ and $Q$ are related as