What is the primary product of an $S_N1$ reaction on an optically active substance?

The major product of the following reaction is:

Tertiary alkyl halides are practically inert to substitution by the $S_N2$ mechanism due to:

The major product obtained in the above reaction is

Which of the following undergoes $S_{N^{1}}$ reaction?

Under the specified conditions,substrate $X$ undergoes substitution and elimination reactions to give products $A - D$. $A$ and $B$ are stereoisomers,but not enantiomers. $C$ and $D$ are enantiomers. $A$ is not an isomer of $C$. Which of the following could be the starting material $X$ ?
$X \xrightarrow[\Delta]{H_2O} A + B + C + D$