Which of the following alkyl-substituted alkenes is the most stable?

Compound $X$ on reaction with $O_{3}$ followed by $Zn/H_{2}O$ gives formaldehyde and $2-$methylpropanal as products. The compound $X$ is :

Ozonolysis of which compound gives $OHC-CH_2-CH(CH_3)-CH_2-CHO$?

Final product formed on reduction of glycerol by hydroiodic acid is

Reagent $A$ may be:

What is the product obtained when an alkene is reacted with cold and dilute alkaline potassium permanganate $(KMnO_4)$?