Stress σ is proportional to:

Stress (σ) is defined as the force per unit area applied to an object. It is a fundamental concept in mechanics and materials science. The relationship between stress and strain is described by Hooke's Law, which states that, within the elastic limit of a material, stress is proportional to strain.

Strain is a measure of deformation representing the displacement between particles in a material body. When a material is subjected to stress, it will deform (or strain) in response to that stress. The proportional relationship between stress and strain allows engineers and materials scientists to predict how a material will behave under various loading conditions, which is crucial for ensuring the reliability and safety of structures and mechanical components.

The other options, while related to the field of mechanics, do not capture this specific relationship as directly as stress and strain do. Force, while it directly contributes to stress, is not the same as stress itself. Pressure is another form of stress but is typically used in the context of fluids. Therefore, the most accurate representation of the proportional relationship in this context is between stress and strain.