The correct option is A SN1 mechanism
The mixture containing two enantiomers in equal proportions will have zero optical rotation, as the rotation due to one isomer will be cancelled by the rotation due to the other isomer. Such mixture is racemic modification. The process of conversion of enantiomer into a racemic mixture is called as racemisation. In case of optically active alkyl halides SN1 reactions are accompanied by racemisation. This is because the carbocation formed in slow step is planar.