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

Optical isomerism arises due to the presence of

Which of the following cyclopentane derivatives is optically inactive?

Which of the following does not exhibit optical isomerism?

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 of the following is an optically active molecule?