Decarboxylation

yes to all who say that carbanion is the intermediate ! bridgehead carbanion is the most unstable in this context...hence (a) never undergoes decarboxylation !
@msp...you should not have pinked that post without confirmation...gives others a wrong concept here !

Yeah, I think carbonanion will be formed in this case.

well eure thermolysis is a way to produce free radicals,i confirmed it in the peter sykes,(free radical chapter).

eure remove the + sign and put a dot in that place,nothing more.and we know the free radical is sp2 hybridised as like a carbocation.

can u explain ur point with a figure msp ??

actually i think free radical will be formed, and not carbocation formation.Neways the free radical formed shud be sp2hybridised,and it shud have a planar structure,so the intermediate is extremely unstable.

if decarboxylation occurs in a) , then -CO₂ would be eliminated forming bridgehead carbanion as transition intermediate , which is very unstable.

thus answer is a.

ans is a..