Which of the following acyclic compounds will react fastest with $Br_2$ gas?

$A$ 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:

$CH_3-CH=CH_2$ $\xrightarrow{Br_2, hv (\text{low conc.})} \dots$ $\xrightarrow{Mg, \text{Dry ether}} \dots$ $\xrightarrow{CH_3-CO-CH_3, NH_4Cl} \dots$ $\xrightarrow{H^{+}, \Delta} (X)$. The end product $(X)$ of the above reaction is:

Ozonolysis of an alkene with molecular formula $C_9H_{18}$ yields $2, 2-$ dimethylpropanal and $2-$ butanone. The alkene is ....................

The reaction of $2-$methylbuta$-1,3-$diene with $O_3$ followed by $Zn-H_2O$ gives:

The reaction of $1-$ethylcyclopentene with $BH_3 / THF$ followed by treatment with $H_2O_2 / NaOH$ produces: