Which is the correct structure of $D$-glyceraldehyde?

Which of the following pairs of compounds are not positional isomers?

The correct statement$(s)$ concerning the structures $E, F$ and $G$ is (are):
$E$: $CH_3CH_2CH(CH_3)COCH_3$
$F$: $CH_3CH_2C(OH)=C(CH_3)_2$ (enol form)
$G$: $CH_3CH_2C(CH_3)=C(OH)CH_3$ (enol form)
$(A)$ $E, F$ and $G$ are resonance structures
$(B)$ $E, F$ and $E, G$ are tautomers
$(C)$ $F$ and $G$ are geometrical isomers
$(D)$ $F$ and $G$ are diastereomers

The number of stereoisomers possible for a compound of the molecular formula $CH_3-CH=CH-CH(OH)-CH_3$ is:

According to $CIP$ sequence rule,the correct arrangement in order of decreasing priority is :-

Which of the following compounds has a center of symmetry?