1. is because the nucleophile attaches itself on the side opposite to the one where the halogen atom is present.
2. Equimolar mixture of ‘d’ and T forms of an optically active compound is called racemic mixture.
3. Racemic mixture is optically inactive due to external compensation. A 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.