The correct option is 0⁰.

Ethane molecules exhibit conformational isomerism due to the rotation about the carbon-carbon single bond.

The two main conformers of ethane are the staggered conformer and the eclipsed conformer:

1. Eclipsed Conformation:
In this conformation, the hydrogen atoms attached to one carbon atom are directly aligned with (or "eclipse") the hydrogen atoms attached to the adjacent carbon atom when viewed along the C-C bond axis. This alignment causes maximum electrostatic repulsion between the electron clouds of the C-H bonding pairs, resulting in high torsional strain. Therefore, the eclipsed conformer is the least stable conformation of ethane.

In this least stable (eclipsed) conformation, the dihedral angle (the angle between C-H bonds on adjacent carbons) is:

$\theta =0°$

2. Staggered Conformation:
In this conformation, the hydrogen atoms on adjacent carbons are as far apart as possible, which minimizes torsional strain and makes it the most stable conformation of ethane.

In this most stable (staggered) conformation, the dihedral angle is:

$\theta =60°$

Therefore, the dihedral angle of the least stable conformer of ethane is 0⁰.