(i) ${\mathit{S}}_{\mathit{N}}2$ 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 ${\mathit{S}}_{\mathit{N}}2$ reaction faster than (b).
(ii) Out of ${\mathit{S}}_{\mathit{N}}1$ and ${\mathit{S}}_{\mathit{N}}2$, which reaction occurs with:
(a) Inversion of configuration: ${\mathit{S}}_{\mathit{N}}2$, because the incoming nucleophile attacks from the backside resulting in inversion of configuration.
(b) Racemisation: ${\mathit{S}}_{\mathit{N}}1$, in this first carbocation is formed.
The incoming nucleophile can attack from either side resulting in racemic mixture.