Which of the following conformations of $n$-butane is the most stable?

The following eclipsed form of propane is repeated after rotation by an angle of $......^o$.

In the following,the most stable conformation of $n-$butane is:

Which of the following hydrides is capable of showing conformations?

What,if anything,can be said about the magnitude of the equilibrium constant $K$ for the following equilibrium?

On another planet,the gauche and anti forms of $1, 2-$dichloroethane freeze and single bond rotation stops. Under these conditions,these two forms can be considered as: