Which one of the following quantities has no unit?StrainSurface TensionStressViscosity

To determine which of the given quantities has no unit, let's analyze each option:

1. Strain: Strain is defined as the ratio of change in length to the original length. Since it is a ratio of two lengths, it is dimensionless and thus has no unit.

2. Surface Tension: Surface tension is defined as the force per unit length, typically expressed in units such as Newtons per meter (N/m).

3. Stress: Stress is defined as force per unit area. It has units such as Pascals (Pa), where $1 \, \text{Pa} = 1 \, \text{N/m}^2$.

4. Viscosity: Viscosity is a measure of a fluid's resistance to deformation, typically expressed in units such as Pascal-seconds (Pa·s) or poise (P).

Final Answer

Among the choices listed, Strain is the quantity that has no unit.