The $\mathrm{IUPAC}$ name of  is

 Arrange the following free radicals in order of decreasing stability:

$\mathrm{Methyl}\left(\mathrm{I}\right), \mathrm{Vinyl}\left(\mathrm{II}\right),\mathrm{Allyl}\left(\mathrm{III}\right) \mathrm{and} \mathrm{benzyl}\left(\mathrm{IV}\right)$

Bond length of ethane $\left(\mathrm{I}\right),$ ethene $\left(\mathrm{II}\right),$ ethyne $\left(\mathrm{III}\right)$ and benzene $\left(\mathrm{IV}\right)$ follows the order: