The circle represents the most basic atom in these molecules. Which of the following is the correct representation?

Match the following pairs in Column-$I$ with their respective isomer types in Column-$II$.

Column-$I$	Column-$II$
$A$. Acetaldehyde,vinyl alcohol	$1$. Enantiomers
$B$. Eclipsed and staggered ethane	$2$. Tautomers
$C$. $(+)$$2$-butanol,$(-)$$2$-butanol	$3$. Chain isomers
$D$. Methyl-$n$-propylamine and diethylamine	$4$. Conformational isomers
	$5$. Metamers

The hydrogen indicated by the arrow will be easily removed as:

$A$ pair of stereoisomers might be classified in various ways. Which of the following statements are true with respect to pairs of stereoisomers?
$a$. They might be configurational isomers
$b$. They might be diastereomers
$c$. They might be constitutional isomers
$d$. They might be tautomers
$e$. They might be conformational isomers
$f$. They might be enantiomers
$g$. They might be positional isomers

$CH_3-CH=O$ $(I)$ $\rightleftharpoons$ $CH_2=CH-OH$ $(II)$
Between the two tautomers,which is more stable?

Determine the absolute configurations of the labeled carbons ($a$ and $b$).