Which of the following compounds is optically active?

Which one of the following acids does not exhibit optical isomerism?

What is the number of optical isomers for $2, 3-$dibromobutanoic acid?

How many meso isomers of $C_4H_8Cl_2$ are possible?

The correct statement$(s)$ about the compound $H_3C-CH(OH)-CH=CH-CH(OH)-CH_3$ $(X)$ is(are):
$(A)$ The total number of stereoisomers possible for $X$ is $6$.
$(B)$ The total number of diastereomers possible for $X$ is $3$.
$(C)$ If the stereochemistry about the double bond in $X$ is trans,the number of enantiomers possible for $X$ is $4$.
$(D)$ If the stereochemistry about the double bond in $X$ is cis,the number of enantiomers possible for $X$ is $2$.

Optical isomerism arises due to the presence of