How many optically active stereoisomers are possible for butan-$2,3$-diol?

The separation of a racemic mixture into pure enantiomers is termed as

Which one of the following exhibits enantiomerism?

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

How many stereoisomers does butane-$2,3$-diol have?

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 $2$.