Which one of the following conformations of cyclohexane is chiral?

Arrange the following conformational isomers of $n$-butane in order of their increasing potential energy:

Most stable conformation of $n$-butane is

What do you mean by conformation and conformer (Rotamers)?

In the Newman projection for $2,2$-dimethylbutane,what can $X$ and $Y$ respectively be?
$A$. $H$ and $H$
$B$. $H$ and $C_2H_5$
$C$. $C_2H_5$ and $H$
$D$. $CH_3$ and $CH_3$

Which one of the following conformations of cyclohexane is the least stable?