Meso-tartaric acid is

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

$A$ natural product was isolated in the laboratory and its observed rotation was $+10^{\circ}$. When measured in a $1 \ dm$ sample tube containing $1 \ g$ of compound in $10 \ mL$ of water. What is the specific rotation of this compound?

Which of the following can show optical activity?

The number of optical isomers of an organic compound having $n$ asymmetric carbon atoms will be

How many stereoisomers does butane-$2,3$-diol have?