Step 1: Understanding the Question:
The question tests a basic definition from the theory of elasticity.
We are asked which elastic constant is obtained when hydrostatic (volumetric) stress is divided by volumetric strain.
This constant tells us how much a material resists a change in its volume when it is squeezed equally from every direction.
Step 2: Key Formula or Approach:
When a body is subjected to equal stress from every direction (hydrostatic stress \( \sigma_v \)), it undergoes a change in volume \( \Delta V \) from its original volume \( V_0 \).
The volumetric strain is defined as \( \epsilon_v = \Delta V / V_0 \).
The elastic constant that connects the two is called the bulk modulus.
\[ K = \frac{\sigma_v}{\epsilon_v} = \frac{\text{hydrostatic stress}}{\text{volumetric strain}} \]
Step 3: Detailed Explanation:
Option (A) Bulk modulus: by definition this is exactly the ratio of hydrostatic stress to volumetric strain, so this matches the question directly.
Option (B) Compressibility: this is defined as the reciprocal of bulk modulus, \( 1/K \), which connects strain to stress, not stress to strain, so it is the wrong pairing.
Option (C) Poisson's ratio: this compares lateral strain to longitudinal strain under a uniaxial load, it has nothing to do with hydrostatic loading.
Option (D) Young's modulus: this is the ratio of longitudinal stress to longitudinal strain under a simple axial pull, not a volumetric loading condition.
Final Answer:
The ratio of hydrostatic stress to volumetric strain is called the bulk modulus.
\[ \boxed{\text{Bulk modulus}} \]