Alkenes usually show which type of reaction?

Give the increasing order of stability for the following alkenes based on their heat of hydrogenation $(\Delta H^o)$:
$(I) (CH_3)_2C = C(CH_3)_2$
$(II) CH_3CH_2CH_2CH = CH_2$
$(III) (CH_3)_2C = CHCH_3$
$(IV) CH_3CH_2C(CH_3) = CH_2$

The major product of the reaction is:

The reaction of $HBr$ with the following compound would produce:

Which among the following alkenes is most stable?

Ozonolysis of $but-1-ene$ followed by hydrolysis gives: