Correct Answer - Option 4 : Compressibility

**Explanation**:

**Tenacity:**

The tenacity of a body is represented by the greatest longitudinal stress per unit of cross section that it can bear without rupture.

**Plasticity:**

It is the property of material to undergo a non-reversible change of shape in response to an applied force.

**Viscosity:**

The property which opposes the relative motion between two adjacent layers in a fluid is known as viscosity.

Compressibility (α):

It is the measure of the change in volume or density of matter under the application of pressure. It is given as an inverse of the bulk modulus.

\({\rm{\alpha }} = \frac{1}{{\rm{K}}}\)

Bulk modulus (K):

It is the measure of pressure required to create a change in volume per initial volume.

\({\rm{K}} = \frac{{ - {\rm{dp}}}}{{\frac{{{\rm{dv}}}}{{\rm{v}}}}}\)

The minus sign indicates a decrease in the volume of fluid

**Thus the answer is compressibility.**

Compressible and incompressible flow:

The compressibility of flow is decided based on volumetric strain and not on bulk modulus

\(\frac{{{\rm{dV}}}}{{\rm{V}}} \le 0.05,{\rm{then\ the\ flow\ is\ incompressible}}\)

\(\frac{{{\rm{dV}}}}{{\rm{V}}} > 0.05,\;{\rm{then\ the\ flow\ is\ compressible}}\)