Dihedral angle of least stable conformer of ethane is:

Ethane has two main conformations: Staggered and Eclipsed.The eclipsed conformation is the least stable due to maximum torsional strain (repulsion between electron clouds of $\text{C}-\text{H}$ bonds).In the eclipsed conformation, the $\text{H}$ atoms on adjacent carbons are directly behind one another, meaning the dihedral angle is $0^{\circ }$.