Which of the following has no hindered rotation about carbon-carbon bond?

In alkenes, The p-orbitals are mutually parallel and both the p-orbitals are perpendicular to the plane of the molecule. Rotation of one CH2 fragment with respect to other interferes with maximum overlap of p-orbitals and therefore, such rotations about carbon-carbon double bond(C = C) is restricted. In single bond the energy barrier of rotation is low and it can freely rotate and this leads to conformational isomerism. For example as shown below in ethane.