Which of the following forms of tartaric acid is optically active?

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

If there is no rotation of plane polarised light by a compound in a specific solvent,though thought to be chiral,it may mean that

An aqueous solution containing compounds $A$ and $B$ shows optical activity. $A$ and $B$ are stereoisomers. Which of the following possibilities cannot be correct?

Meso-tartaric acid is

Optical activity is measured by .................