Which type of bond exists between the two carbon atoms of an alkene?

Which one of the following is an electrophilic addition reaction?

The major product of the reaction of $2$-butene with alkaline $KMnO_4$ solution is

In the reaction
$CH_3-CH=CH_2$ $\xrightarrow{Cl_2/500\,^{\circ}C} A$ $\xrightarrow{Na/\text{dry ether}} B$
$B$ is

Identify $Z$ in the following reaction sequence:
${C_2}{H_5}I$ $\xrightarrow{Alc. KOH} X$ $\xrightarrow{Br_2} Y$ $\xrightarrow{KCN} Z$

$A$ benzene derivative did not produce a white precipitate with ammoniacal silver nitrate solution (Tollens' reagent) but decolorized cold dilute alkaline $KMnO_4$ solution (Baeyer's reagent). The compound is: