Which type of isomerism is exhibited by $2,3-$dichlorobutane?

Which of the following is expected to be optically active?

The given molecule has a chiral center. Determine its configuration.

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 $2$.

Non-superimposable mirror images are called.........

The total number of stereoisomers possible for $2,3-dichlorobutane$ is: