The correct option is Option 3: Specific Gravity.

Here is why:

Specific Gravity is the ratio of the density of a substance to the density of a reference substance (usually water at $4^\circ\text{C}$). Because it is a ratio of two identical physical quantities (density divided by density), the units cancel out, leaving it completely dimensionless and unitless.

$$\text{Specific Gravity} = \frac{\rho_{\text{substance}}}{\rho_{\text{water}}}$$

Why the other options have units:

Velocity: Measures the rate of change of position. Its SI unit is meters per second ($m/s$).

Density: Measures mass per unit volume. Its SI unit is kilograms per cubic meter ($kg/m^3$).

Stress: Measures force applied per unit area. Its SI unit is the Pascal ($Pa$), which is equal to Newton per square meter ($N/m^2$).