Lactic acid,in which a methyl group,a hydroxyl group,a carboxylic acid group,and a hydrogen atom are attached to a central carbon atom,shows optical isomerism due to the molecular geometry at the:

The compound that is chiral is

The number of stereoisomers of the compound $CH_3CHBrCHBrCOOH$ is

Which of the following will show optical isomerism?

The Fischer projection of $D$-erythrose is defined by the structure $CHO-C(OH)(H)-C(OH)(H)-CH_2OH$ with both hydroxyl groups on the right. $D$-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

Which dimethylcyclobutane is optically active?