The substance with the lowest viscosity is ClCH₂CH₂Cl.
Viscosity of a fluid is its resistance to deformation , or it can be defined as the property that opposes the relative motion between two surfaces of the fluid that are moving at different velocities.
This can be seen by the structure of ClCH₂CH₂Cl, that it is having least viscosity as it is having least number of chlorine atoms that can resisst the motion so it is least viscous.
