Among the following,the conformation that corresponds to the most stable conformation of meso-butane-$2,3$-diol is -

Define torsional strain.

Arrange the following molecules in the correct order of stability (decreasing order):

If the $C_2$ carbon in the given Newman projection is rotated by $120^{\circ}$ in the anticlockwise direction,what will be the resulting conformation of the $C_2-C_3$ bond?

The lowest energy conformer of $n$-butane is

The isomers which can be converted into another forms by rotation of the molecules around single bond are