Which of the following is optically active?

How many meso isomers are possible for $2, 3, 4-\text{pentanetriol}$?

Which type of isomerism is exhibited by $2,3-$dichlorobutane?

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

The well-known compounds, $(+)$-lactic acid and $(-)$-lactic acid, have the same molecular formula, $C_{3}H_{6}O_{3}$. The correct relationship between them is

Which of the following compounds is not chiral?