(i) A adds HBr and its formula C5H10 suggests it to be an alkene.
(ii) A on hydrogenation gives 2-methyl butane and thus, it has the chain of:
(iii) Addition of HBr on A gives B, which on treating with AgOH gives an alcohol C, which on oxidation gives a ketone D. Thus, C is secondary alcohol and B is secondary bromide which can be obtained only when A is