Dihedral angle of least stable conformer of ethane is :

The correct option is 0° (represented in the options as "0⁰").

Ethane ($C_2{H}_6$) exhibits conformational isomerism due to the free rotation about the carbon-carbon ($C-C$) single bond. As the two methyl groups rotate relative to one another, the spatial arrangement of the hydrogen atoms changes, leading to different conformations. The angle between the $C-H$ bonds on adjacent carbon atoms when looking down the $C-C$ bond axis is known as the dihedral angle (or torsional angle).

The two extreme conformations of ethane are:
1. Eclipsed Conformation: In this conformation, the hydrogen atoms on the front carbon are aligned directly in front of the hydrogen atoms on the rear carbon. This alignment minimizes the distance between the hydrogen atoms, leading to maximum electrostatic repulsion between the electron clouds of the $C-H$ bonding pairs (known as torsional strain). Because of this high torsional strain, the eclipsed conformation has the highest potential energy, making it the least stable conformer of ethane. In this fully aligned state, the dihedral angle is exactly $0°$.
2. Staggered Conformation: In this conformation, the hydrogen atoms on the adjacent carbons are as far apart as possible, minimizing torsional strain and maximizing stability. The dihedral angle in the staggered conformation is $60°$.

Thus, the least stable conformer of ethane is the eclipsed conformer, and its dihedral angle is $0°$.