CBSE Class 12 · 2016 · All India · Haloalkanes and HaloarenesVery Short Answer

In an S${}_{\mathrm{N}}$2 reaction the nucleophile attacks from the side opposite the leaving group, so the configuration at the chiral carbon inverts (Walden inversion).
Of the two displayed reactions, reaction (i) gives the product of inverted configuration — hence \textbf{reaction (i) is S${}_{\mathrm{N}}$2}.
(Reaction (ii), proceeding with racemisation via a planar carbocation, is S${}_{\mathrm{N}}$1.)