The marked bond is the weakest bond because it is longer than the other bonds, and cleavage of this bond results in the formation of very stable bridgehead alkyl radicals.

The marked bond is the weakest bond because it is longer than the other bonds, and cleavage of this decreases the ring strain.

The marked bond is the weakest bond because it is shorter than the other bonds, and cleavage of this bond results in the formation of very stable bridgehead alkyl radicals.

The marked bond is the weakest bond because it is shorter than the other bonds, and cleavage of this decreases the ring strain.