If the group $CH_3-CH(CH_3)-CH_2-CH(CH_3)-$ is present as a branch in a molecule,which of the following names is correct? $(i)$ $2, 4-$dimethylbutyl $(ii)$ $1, 3-$dimethylbutyl

(B) The correct name is $1, 3-$dimethylbutyl.
The numbering of the substituent group starts from the carbon atom that is directly attached to the parent chain,which is assigned the number $1$.
Therefore,the methyl group at the first carbon and the methyl group at the third carbon result in the name $1, 3-$dimethylbutyl.