Which of the following compounds has a center of symmetry?

The total number of stereoisomers possible for the following compound is:

$A$ 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

Which of the following Fischer projection formulas is the same as $D$-Glyceraldehyde?

Which one of the following pairs represents stereoisomerism?

The number of stereoisomers possible for $1,2-$dimethylcyclopropane is: