Which reaction will yield $CH_2 = C = CH_2$?

"The negative part of the addendum adds on to the carbon atom linked with the least number of hydrogen atoms." This statement is called:

Two isomers $A$ and $B$ with molecular formula $C_4H_8$ give different products on oxidation with $KMnO_4$ in acidic medium. Isomer $A$ on reaction with $KMnO_4 / H^{+}$ results in effervescence of a gas and gives a ketone. The compound $A$ is ...... .

Which of the following reagents is used to test for the unsaturation of an alkene?

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

What is the product obtained from the reaction of propene with $HCl$,$HBr$,and $HI$ in the presence of peroxide?