(i) SN2 reaction occurs more quickly in primary alkyl halides than in secondary and tertiary alkyl halides.
(a) is primary alkyl halide and (b) is secondary alkyl halide.
So, (a) undergoes SN2 reaction faster than (b).
(ii) Out of SN1 and SN2, which reaction occurs with:
(a) Inversion of configuration: SN2, because the incoming nucleophile attacks from the backside resulting in inversion of configuration.
(b) Racemisation: SN1, in this first carbocation is formed.
The incoming nucleophile can attack from either side resulting in racemic mixture.