What type of reaction is oxidative rancidity?

Consider the following carbocations:
$I. C_6H_5CH_2^+$
$II. CH_2=CH^+$
$III. CH_3-CH^+(CH_3)$
$IV. CH_3-CH_2^+$
$V. HC \equiv C^+$
Arrange the above carbocations in the order of decreasing stability.

In the potential energy diagram shown,the point $X$ represents:

The correct order of stability of the following carbocations is:

Which of the following molecules/species is the most stable?

In the given compound,$Cl$ will be liberated easily in the form of: