Why is $Meso-tartaric$ acid optically inactive?

Which of the following compounds will exhibit optical isomerism?

Which of the following molecules with the molecular formula $C_4H_8O$ exhibits optical isomerism?

The configurations of the carbon atoms $C_2$ and $C_3$ in the following compound are respectively:
$COOH-CH(OH)-CH(OH)-CHO$

Which among the following compounds has a chiral carbon atom?

The concentration of an organic compound in chloroform is $6.15 \ g$ per $100 \ mL$ of solution. $A$ portion of this solution in a $5 \ cm$ polarimeter tube causes an observed rotation of $-1.2^{\circ}$. What is the specific rotation of the compound?