The S_N1 reaction is not possible because a bridgehead carbocation can not be formed due to high angle and ring strains.
The S_N2 reaction is not possible because of the high steric hindrance for the backside attack of the nucleophile.

The S_N1 reaction is not possible because tertiary benzylic carbocations are not stable.
The S_N2 reaction is not possible because of the high steric hindrance for the backside attack of the nucleophile.

The S_N1 reaction is not possible because the resulting carbocation would be in conjugation with the ring.
The S_N2 reaction is not possible because the incoming nucleophile would attack the benzene ring instead.

The S_N1 and S_N2 reactions are not possible because iodide is a poor leaving group.