Choose the most reasonable reaction intermediate for the following reaction.

The stability of carbanions follows the order:
$I: CH_3CH_2CH_2\bar{C}H_2$
$II: CH_3\bar{C}HCH_2CH_3$
$III: (CH_3)_3\bar{C}$
$IV: CH_3\bar{C}(Ph)CH_2CH_3$

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 carbocations will be most stable?

The correct order of increasing stabilities of the following carbocations is:
$(i)$ Allyl carbocation
$(ii)$ Ethyl carbocation
$(iii)$ Benzyl carbocation
$(iv)$ Isobutyl carbocation

Which of the following cannot exist at room temperature?