Which compound on oxidation with acidic $KMnO_4$ gives Benzoic acid?

The major products $Q$ and $R$ from the following reactions,respectively are:

Compound $A$ was treated with a large excess of $CH_3MgBr$. The resulting product was exposed to $POCl_3$ /pyridine to give compound $B$,as one of many products.
Which of the following compounds can be $A$ ?

Hydrolysis of an ester gives a carboxylic acid which on Kolbe's electrolysis yields ethane. The ester is

Give plausible explanation for each of the following:
$(i)$ Cyclohexanone forms cyanohydrin in good yield but $2,2,6-$trimethylcyclohexanone does not.
$(ii)$ There are two $-NH_2$ groups in semicarbazide. However,only one is involved in the formation of semicarbazones.
$(iii)$ During the preparation of esters from a carboxylic acid and an alcohol in the presence of an acid catalyst,the water or the ester should be removed as soon as it is formed.

At which value of $n$ does the formation of a six-membered ring take place in the following reaction?
$Br-(CH_2)_n-CH_2-Br + CH_2(CO_2Et)_2 \xrightarrow{NaOEt, EtOH} \text{cyclic product}$