Among the structures given, select the enantiomers.

The total number of chiral molecules formed from one molecule of $P$ on complete ozonolysis $(O_3, Zn / H_2O)$ is. . . . . . .

The total number of stereoisomers that can exist for $M$ is:
$M$ is a bicyclic ketone with a methyl group at the bridgehead and two methyl groups at the bridge carbon.

What is the process of conversion of a pure optical enantiomer into an optically inactive mixture of $(+)$ and $(-)$ enantiomers called?

The Fischer projection of $D$-erythrose is shown below.
$D$-Erythrose and its isomers are listed as $P$,$Q$,$R$,and $S$ in Column-$I$. Choose the correct relationship of $P$,$Q$,$R$,and $S$ with $D$-erythrose from Column-$II$.
Column-$I$:
$P$: $CHO-C(OH)(H)-C(OH)(H)-CH_2OH$ (rotated)
$Q$: $CHO-C(H)(OH)-C(OH)(H)-CH_2OH$
$R$: $CHO-C(OH)(H)-C(H)(OH)-CH_2OH$
$S$: $CHO-C(H)(OH)-C(H)(OH)-CH_2OH$ (enantiomer)
Column-$II$:
$1$. Diastereomer
$2$. Identical
$3$. Enantiomer

$CH_3-CH(OH)-COOH$ shows