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

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

The molecules represented by the two structures shown in the image are:

$A$ 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

$A$ naturally occurring substance has the constitution shown below. How many stereoisomers may have this constitution?

Identify the conformer of $2$-methylpentane.