D

Step 1: Understand the types of elastic modulus.
The elastic moduli describe how materials deform under stress. For solids, these include Young's modulus, shear modulus, and modulus of rigidity. However, fluids behave differently than solids.

Step 2: Identify the relevant modulus for fluids.
In the case of fluids, they cannot sustain shear stress and hence do not exhibit shear modulus or Young's modulus. The relevant property for fluids is the bulk modulus, which measures a material's response to uniform pressure applied from all directions.

Step 3: Definition of Bulk Modulus.
The bulk modulus (K) is defined as the ratio of the change in pressure to the fractional change in volume:
$$ K = -V \frac{\Delta P}{\Delta V} $$
where $V$ is the original volume, $\Delta P$ is the change in pressure, and $\Delta V$ is the change in volume.

Conclusion: Since fluids do not support shear stress, the only modulus applicable to them is the bulk modulus. Therefore, the correct answer is D.