Which of the following compounds is achiral?

The number of stereoisomers for a compound containing $n$ different asymmetric carbon atoms will be .............

Which of the following combinations amongst the four Fischer projections represents the enantiomers?

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 two enantiomers of secondary butyl chloride differ from each other in which one of the following properties?

How many optically active isomers are possible for the compound with the molecular formula $C_4H_9Br$?