(A)

In ${\mathrm{S}}_{\mathrm{N}}1$ reaction, carbocation is formed as an intermediate. Carbocation is planar and nucleophile may attack from either side of the plane. This gives a pair of enantiomers as reaction products i.e. product is racemic mixture. Although amount of inversion product is more than that of retention product because nucleophile starts attacking from back side even before leaving group has completely left the carbon. So in ${\mathrm{S}}_{\mathrm{N}}1$ there is partial racemization.

In ${\mathrm{S}}_{\mathrm{N}}2$, there is 100% inversion because nucleophile attacks from the side opposite to leaving group.

(D)

Secondary alkyl halide follows both ${\mathrm{S}}_{\mathrm{N}}1$ and ${\mathrm{S}}_{\mathrm{N}}2$ paths parallelly. Out of ${\mathrm{S}}_{\mathrm{N}}1$ and ${\mathrm{S}}_{\mathrm{N}}2$, which one is preferred path by secondary alkyl halide depends upon other conditions such as strength of nucleophile and nature of solvent. Strong nucleophiles such as alkoxides and polar aprotic solvents such as DMF and DMSO prefer to keep the reaction along ${\mathrm{S}}_{\mathrm{N}}2$ pathway. Weak nucleophiles such as water, alcohols and acetic acid and polar protic solvents such as water and alcohols (these are solvents as well as nucleophiles), prefer to keep the reaction along ${\mathrm{S}}_{\mathrm{N}}1$ pathway.

(A)

Stereospecific reactions are those reactions for which if stereochemistry of reactants is changed, then major product formed is changed. In ${\mathrm{S}}_{\mathrm{N}}2$ reactions, inversion occurs, so different stereoisomers give different products. Hence ${\mathrm{S}}_{\mathrm{N}}2$ reactions are stereospecific.

In ${\mathrm{S}}_{\mathrm{N}}1$ reactions, carbocation is formed as intermediate. Carbocation is planar hence nucleophile can attack from either side of the planar carbocation. This results in racemic mixture in products.