Alkyl iodides on keeping become dark in color due to ...........

In the following reactions,
$(i) CH_3-CH(CH_3)-CH(OH)-CH_3 \xrightarrow{H^{+}/\text{heat}} A \text{ [Major product]} + B \text{ [Minor product]}$
$(ii) A$ $\xrightarrow[\text{in absence of peroxide}]{HBr, \text{dark}} C \text{ [Major product]} + D \text{ [Minor product]}$
the major products $(A)$ and $(C)$ are respectively:

Which of the following alkyl halides is the fastest to undergo dehydrohalogenation with a base?

Which compound has the highest reactivity towards $S_{N}1$ reaction?

The major product obtained in the reaction of $1$-phenyl-$2$-bromobutane with $NaOMe$ is:

The product $(Y)$ is: