Isomerism due to rotation around the $C-C$ single bond is known as:

Given below are two statements:
Statement $I$: There are several conformers for $n$-butane. Out of those conformers, the fully eclipsed conformer (dihedral angle $0^{\circ}$) is the least stable and the anti-staggered conformer (dihedral angle $180^{\circ}$) is the most stable.
Statement $II$: As the dihedral angle increases from $0^{\circ}$ to $60^{\circ}$, torsional strain decreases from the fully eclipsed conformer $(X)$ to the gauche conformer $(Y)$.
In the light of the above statements, choose the correct answer from the options given below:

The Newman projection shown below is known as the:

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

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

Given structures $I$ and $II$ are: