Define hyperconjugation by taking an example of propene.

The correct stability order of the following resonating structures is:

${\mathrm{CH}}_2\underset{\left(\mathrm{I}\right)}{=}\mathrm{C}=\mathrm{O}$,  ${\mathrm{H}}_2\stackrel{-}{\mathrm{C}}\underset{\left(\mathrm{II}\right)}{-}\stackrel{+}{\mathrm{C}}=\mathrm{O}$,  ${\mathrm{H}}_2\mathrm{C}\stackrel{}{\underset{\left(\mathrm{III}\right)}{=}}\stackrel{+}{\mathrm{C}}-\stackrel{-}{\mathrm{O}}$, ${\mathrm{H}}_2\stackrel{-}{\mathrm{C}}\underset{\left(\mathrm{IV}\right)}{-}\mathrm{C}\equiv \stackrel{}{\mathrm{O}}$