The correct option is C 180o
Dihedral angle or torsional angle is the angle between planes through two sets of three atoms, having two atoms in common.
Although the technically correct way would be to look at the planes, for our ease we can just look at the bonds directly in most cases.
So, in this case, the configuration is Cl-C-Cl. Angle will be 180o. This is called the anti-conformation.