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

Which compound is chiral?

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 $2$.

The number of optical isomers of an organic compound having $n$ asymmetric carbon atoms will be

Which of the following are enantiomers?

Which of the following exhibits optical isomerism?