Alkenes usually show which type of reaction?

Addition of $HCl$ to $3,3$-dimethyl-$1$-butene yields two products,one of which has a rearranged carbon skeleton. Among the following carbocations,select the possible intermediates in that reaction:
$1.$ $(CH_3)_3C-C^+H-CH_3$
$2.$ $(CH_3)_3C-CH_2-C^+H_2$
$3.$ $(CH_3)_2C(Cl)-C^+(CH_3)_2$
$4.$ $(CH_3)_2C^+-CH(CH_3)_2$

What product is formed when the alkene $R-CH=CH_2$ reacts with $B_2H_6$ in the presence of $H_2O_2$?

Which of the following alkenes gives both $cis$ and $trans$ isomers upon catalytic hydrogenation?

Which reaction of ethanol will decolourise bromine water?

Major product of the given reaction is $CH_2=CH-CH_2-I \xrightarrow[CCl_4]{HI(excess)}$