Which of the following has maximum stability?

Consider the following compounds:
$(I)$ $(CH_3)_3C-CH^{\bullet}-C_6H_5$
$(II)$ $(C_6H_5)_3C^{\bullet}$
$(III)$ $2$-methylbicyclo$[2.2.1]$hept$-2-$yl radical (as shown in the image)
Hyperconjugation occurs in:

Which of the following cannot exist at room temperature?

Carbocation as an intermediate is likely to be formed in the reaction:

With a change in hybridisation of the carbon bearing the charge,the stability of a carbanion increases in the order:

Which of the following is the least stable carbon free radical?