Dihedral angle of least stable conformer of ethane is :

In ethane (C₂H₆) the carbon–carbon single bond allows free rotation. The relative orientation of the two CH₃ groups is described by the dihedral (torsion) angle θ measured between a C–H bond on the front carbon and the nearest C–H bond on the back carbon.

When θ = 0° the two CH₃ groups are **eclipsed**: each C–H bond on the front carbon lies directly behind a C–H bond on the back carbon. This alignment brings the electron clouds of the four hydrogen atoms into close proximity, creating strong repulsive interactions (torsional strain). Consequently the potential energy is at a maximum.

When the molecule rotates to a **staggered** arrangement (θ = 60°, 180°, 300°), each C–H bond on the front carbon points between two C–H bonds on the back carbon. The repulsive interactions are minimized, so the energy is lower. Among the staggered conformations, the one at 60° (and its equivalents) is the most stable because it also avoids any additional steric crowding.

Thus, the conformer with the highest energy—and therefore the least stable—is the eclipsed conformer that occurs at a dihedral angle of $0°$. This is why the correct answer is $0°$.