The correct answer is option 'A' - Modulus of elasticity.

Explanation:
Modulus of elasticity is defined as the ratio between stress and strain. It is a measure of a material's stiffness or ability to deform under stress. It quantifies how much a material will stretch or compress when subjected to a given amount of stress. The modulus of elasticity is an important property that helps engineers and scientists understand the behavior of materials under different loading conditions.

Let's understand the concept of stress and strain and how they are related to the modulus of elasticity:

1. Stress:
Stress is defined as the force applied per unit area. It is a measure of the internal resistance of a material to deformation under external forces. Stress is calculated by dividing the applied force by the cross-sectional area of the material. The formula for stress is:

Stress = Force / Area

2. Strain:
Strain is the measure of the deformation or elongation of a material due to stress. It is calculated by dividing the change in length of a material by its original length. The formula for strain is:

Strain = Change in length / Original length

3. Modulus of Elasticity (Young's Modulus):
The modulus of elasticity, also known as Young's modulus, is defined as the ratio of stress to strain within the elastic limit of a material. It represents the slope of the stress-strain curve in the linear elastic region. The formula for Young's modulus is:

Young's modulus = Stress / Strain

Since stress and strain are directly proportional within the elastic limit, the ratio of stress to strain remains constant. This constant ratio is known as the modulus of elasticity.

In conclusion, the ratio between stress and strain is called the modulus of elasticity. It is denoted by the symbol 'E' and is a fundamental property of materials that characterizes their stiffness.