Which of the following contains an asymmetric centre?

What is the correct configuration of the following molecule?

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 $2$.

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

Which one of the following compounds shows optical isomerism?

$A$ compound has $3$ chiral carbon atoms. The number of possible optical isomers it can have is: